Vibration Generalized Eigenvalue Problem (GEP) helper functions.

Targeted at matrix pencils with real symmetric matrices. The GEP reads

(K - omega^2 * M) * v = 0

Here K is a symmetric, positive semi-definite, stiffness matrix, M is a symmetric positive definite mass matrix, omega is the angular velocity.

- Solve the vibration GEP for the smallest eigenvalues:

```
d, v, nconv = gep_smallest(K + omega_shift^2 * M, M, neigvs; method = :Arpack)
```

The solution is useful in constructing modal expansions in solid dynamics. It is
possible to select the method (package) to use, `:KrylovKit`

, `:Arpack`

, `:ArnoldiMethod`

, and
`:SubSIt`

are currently available.

- Solve the vibration GEP for the largest eigenvalue:

```
omega_max = gep_largest(K, M)
```

The solution is useful in estimating the stable time step in explicit integration of the equations of motion.