Add support for tensor network forms (bilinear and quadratic)

[JoeyT1994]

This PR adds the BilinearForm and QuadraticForm types along with the AbstractForm

We define an AbstractForm to be of the structure <x|A|y> where x, y are Tensor Network States and A is a Tensor Network Operator. The AbstractForm contains the mapping from the vertices of the original states to their names in the Form, along with the mapping from the vector space to the dual space.

The BilinearForm is the generic case where x, y and A are all distinct and constructors are defined for this case.
The QuadraticForm is the case when x and y are identical and the constructor definitions reflect that. In this case functions like gradient(qf::QuadraticForm, state_vertices::Vector) are defined for getting the derivative of the qf with respect to the vertices of the original state x.

A short series of tests are included.

[codecov-commenter]

Codecov Report

Attention: 32 lines in your changes are missing coverage. Please review.

Comparison is base (da7636e) 73.24% compared to head (e3010a5) 73.25%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/formnetworks/abstractformnetwork.jl	50.00%	20 Missing ⚠️
src/formnetworks/quadraticformnetwork.jl	66.66%	7 Missing ⚠️
src/formnetworks/bilinearformnetwork.jl	76.19%	5 Missing ⚠️

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Additional details and impacted files

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##             main     #136   +/-   ##
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  Coverage   73.24%   73.25%           
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  Files          68       70    +2     
  Lines        4033     4101   +68     
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+ Hits         2954     3004   +50     
- Misses       1079     1097   +18     

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[mtfishman]

@JoeyT1994 you marked a number of comments as resolved but then I don't see the corresponding changes made.

[JoeyT1994]

@mtfishman I think something has gone out of sync with Github because they are resolved on my local machine. I think it keeps using .jl files from FormNetworks before it was renamed to formnetworks. Let me try to resolve it

[mtfishman]

@JoeyT1994 anything left to do here besides changing the directory FormNetworks back to formnetworks?

[JoeyT1994]

I don't think so, at least for now. Just trying to change the folder name, git is very strange about renaming case sensitive things so I think I need to use some extra commands

[JoeyT1994]

Okay renaming dealt with

[mtfishman]

Looks good. I think this already takes us pretty close to being able to use general tensor networks in alternating_update!