Implement apply_stress for Laplace based Stokes/Elasticity wrapper

[isuruf]

This can be done in the naive way without using Laplace kernels and would be no
different than naive/biharmonic methods.
However since the Laplace based ones are supposed to improve the performance by
reducing the number of FMMs, we should figure out if there's a way to implement this
using Laplace.

cc @alexfikl