Implement kernel rewriting

[alexfikl]

This implements rewriting a target kernel (e.g. Stokeslet) in terms of a base kernel (e.g. biharmonic) as a linear constant-coefficient combination of derivatives.

Currently, it just hoists get_deriv_relation_kernel from inducer/pytential#162 with some minimal changes:

• Clean up the retry logic for the LU factorization.
• Add a loop on the max order of derivatives.

TODO:

• Make sure the LU based decomposition works the same as before.
• Add a bunch of tests for it (based on the tests in pytential).
• Implement a hopefully better (but likely slower?) purely-symbolic version.

cc @isuruf