Nonlinear eigensolver

[simlapointe]

Adding nonlinear eigensolver capability to Palace to enable eigenmode simulations with frequency-dependent boundary conditions such as WavePort, Conductivity, or second-order Absorbing.

Specifically, two nonlinear eigensolver options are introduced:

• Hybrid (default): a hybrid approach where the nonlinear term is (coarsely) approximated with a second-order polynomial, the quadratic EVP is solved with the existing SLEPc or ARPACK polynomial eigensolvers, and the resulting eigenmodes are used as initial guesses for a (deflated) quasi-Newton eigenvalue solver.
• SLP: SLEPc's Successive Linear Problem (SLP) solver from the NEP interface.

Three eigenmode regression tests are added:

• CPW with lumped ports and second-order absorbing BCs.
• CPW with wave ports.
• Adapter waveguide (rectangle at one end, ellipse at the other)

Closes #422.