Add basic setup to solve PDEs by collocation

[JoshuaLampert]

Add feature to solve PDEs by collocation. Here, only asymmetric collocation is used (Kansas method). Symmetric collocation will follow in a future PR. To avoid writing out all derivatives of the kernels by hand, we use AD to compute them (which is less performant, but easier to implement and easier to extend). A simple example solving the Poisson equation with the Wendland kernel is added. More PDEs will follow in future PRs.
The documentation of this new feature will also follow once this is a bit more mature.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (d90d905) to head (f69cdfe).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #36      +/-   ##
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+ Coverage   98.39%   98.54%   +0.14%     
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  Files          21       24       +3     
  Lines         687      755      +68     
==========================================
+ Hits          676      744      +68     
  Misses         11       11              

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