Feature Request: Eigenvalue solver

The `dolfinx_mpc.LinearProblem` class provides a fantastic, easy-to-use wrapper for using mpc without the user having to know much about PETSc or MPI. I would love if there were a similar class that solves the eigenvalue/eigenvector problem the same way. Such a solution would have to use something like SLEPc, which I think would be perfect.

This is important for finding resonant modes for structures that are composed of multiple bonded parts.