Separate ElasticityKernel and Stokeslet

[alexfikl]

This makes the StokesletKernel an ExpressionKernel instead of inheriting from ElasticityKernel. Besides that, it also

1. Removes the __new__ implementations. This is breaking (since ElasticityKernel no longer returns a Stokeslet in special cases).
2. Deprecates passing a non-string value for viscosity_mu and poisson_ratio to match the existing scalar kernels.
3. Renames viscosity_mu and poisson_ratio to viscosity_mu_name and poisson_ratio_name.
4. Makes it an error to pass non-string values to these things. This matches all the other kernels.

[alexfikl]

I think this is right, i.e. it's not $-\Delta K = \delta$, but pointing it out just to make sure. 😁

[Copilot]

Pull request overview

This PR refactors the PDE kernel hierarchy by separating StokesletKernel from ElasticityKernel, making StokesletKernel a standalone ExpressionKernel. It also removes custom __new__ behavior and introduces deprecations around passing non-string material parameters to align with other scalar kernels.

Changes:

• Refactor StokesletKernel to inherit directly from ExpressionKernel and implement its own expression/scaling/args/pickling behavior.
• Remove __new__-based class substitution and introduce DeprecationWarnings for non-string viscosity_mu / poisson_ratio.
• Update misc tests to focus on pickle round-trips for elasticity-related kernels.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
sumpy/kernel.py	Separates StokesletKernel from ElasticityKernel, adds deprecation warnings, and expands kernel docstrings/references.
sumpy/test/test_misc.py	Updates a kernel test to validate pickling behavior after the refactor.

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[alexfikl]

@inducer This should also be good for a look (the CI passed once). It doesn't seem to break pytential, so not bad for all the breaking changes it makes 😁