Add "Used by" (#528)

Add information on how GAlgebra is used in other projects.

This list provides information on GAlgebra's usefulness for some, and these downstream projects can be used to validate the GAlgebra against them.

README.md

@@ -25,6 +25,8 @@ Development Status
 
 The fork supports Python 3, increases test coverage, sets up CI and linters, maintains releases to [PyPI](https://pypi.org/project/galgebra/#history), improves [docs](http://galgebra.readthedocs.io) and has many bug fixes, see [Changelog](https://galgebra.readthedocs.io/en/latest/changelog.html).
 
+For information on how GAlgebra is used in other projects, see [Used by](https://github.com/pygae/galgebra/blob/master/doc/used_by.md).
+
 > [!IMPORTANT] 
 > Readers of Prof. Alan Macdonald's [Linear and Geometric Algebra](http://www.faculty.luther.edu/~macdonal/laga/index.html) and [Vector and Geometric Calculus](http://www.faculty.luther.edu/~macdonal/vagc/index.html), please check out [**Migrating guide for readers of LAGA&VAGC**](#migrating-guide-for-readers-of-lagavagc) below.
 > 

doc/used_by.md

@@ -0,0 +1,48 @@
+# Used by
+
+Here is a list of projects that use GAlgebra, divided into two parts:
+
+1. research: used to facilitate or verify research
+2. functional: used as a dependency in their major functionality
+3. explorational: used as an optional dependency in their examples, exercises, or tutorials
+
+They are roughly ordered by relevance, stars and last updated time.
+
+This data is manually mined from [forks](https://github.com/pygae/galgebra/forks) and [dependents](https://github.com/pygae/galgebra/network/dependents), as ["Used by" button is not shown](https://docs.github.com/en/code-security/supply-chain-security/understanding-your-software-supply-chain/exploring-the-dependencies-of-a-repository#changing-the-used-by-package). All usages are manually reviewed to rule out false positives (e.g. the dependency maybe added, but is not actually used), some usages are manually added to our knowledge.
+
+## Research
+
+(in recent years, non-exhaustive)
+
+- [Geometric Product of Two Oriented Points in Conformal Geometric Algebra (2024)](https://link.springer.com/article/10.1007/s00006-024-01363-6)
+  - two anonymous expert reviewers, who either used the python symbolic geometric algebra implementation GAlgebra or the Clifford python library to independently verify the results of this paper
+- [Machine Learning Clifford invariants of ADE Coxeter elements (2024)](https://arxiv.org/abs/2310.00041)
+  - uses GAlgebra for ADE invariant generation [code](https://github.com/DimaDroid/ML_Clifford_Invariants/blob/main/InvariantData/ADE8_InvariantGeneration.py)
+- [Clifford spinors and root system induction: H4 and the Grand Antiprism (2021)](https://arxiv.org/abs/2103.07817)
+  - perform practical computations in group theory via versors in Clifford algebra framework
+  - LaTeX output of GAlgebra is used in paper [code](https://github.com/ppd22/galgebra-in-action)
+
+## Functional
+
+- Prof. Alan Macdonald's books: [Linear and Geometric Algebra](http://www.faculty.luther.edu/~macdonal/laga/index.html) and [Vector and Geometric Calculus](http://www.faculty.luther.edu/~macdonal/vagc/index.html)
+  - GAlgebra is the books' companion library, see [GAlgebraPrimer](http://www.faculty.luther.edu/~macdonal/GAlgebraPrimer.pdf)
+- [micahscopes/alglbraic](https://github.com/micahscopes/alglbraic): A python library and CLI utility for generating libraries of algebraic GLSL functions, including Clifford algebras
+- [russellgoyder/sundial](https://github.com/russellgoyder/sundial): The sundial problem from a new angle
+  - released as package `analemma` on PyPI
+  - contains [a cheat sheet for GAlgebra](https://github.com/russellgoyder/geometric-algebra-cheat-sheet)
+  - see also https://github.com/pygae/galgebra/issues/506
+- [jdekozak/dirac5d](https://github.com/jdekozak/dirac5d): Five dimensional Dirac equation over the reals
+  - see also https://github.com/jdekozak/dirac5d/issues/4
+- [pygae/GAlgebra.jl](https://github.com/pygae/GAlgebra.jl): Julia interface to GAlgebra
+  - contains test cases for [Eric Chisolm's Geometric Algebra](https://arxiv.org/abs/1205.5935)
+
+## Explorational
+
+- Dr. Eric Wieser's PhD thesis *Formalizing Clifford algebras and related constructions in the Lean theorem prover*
+    - in section 3.3. Symbolic, the approach of GAlgebra is demonstrated and discussed
+- [appliedgeometry/poissongeometry](https://github.com/appliedgeometry/poissongeometry): A Python module for (local) Poisson-Nijenhuis calculus on Poisson manifolds, with the following functions
+  - used in notebook tutorials: https://github.com/appliedgeometry/poissongeometry/blob/master/docs/Tutorial_Ingles.ipynb
+- [meuns/galgebra: GA4CS examples, PGA, code-generation, and more](https://github.com/meuns/galgebra)
+  - see also https://github.com/pygae/galgebra/pull/68
+- [hugohadfield/sym_scratch](https://github.com/hugohadfield/sym_scratch)
+  - used in notebooks for CGA