Subspace iteration algorithm for the generalized eigenvalue problem of free vibration.

Implementation of the Bathe Subspace Iteration algorithms from the textbook Finite Element Procedures (1996), Table 11.3.

A couple of twists have been added, which seem to improve convergence. This package is sometimes slower than Arpack, but generally only 10-20%. Other times it can be faster than Arpack.

With contributions of Michael Stewart.