eigenValuesNonSym: Set size of the output vector before filling it

Rather than assuming the caller has set it to the correct size already.
Setting the size in the method eigenValuesNonSym itself makes code
more robust.  Plus, the time needed for that should be negligible
compared to the actual eigenvalue computations.
Rip out the complete test for eigenValuesNonSym and replace it by the Rosser matrix test from fieldmatrixtest.cc

The old test was complete nonsense: an implementation of some sort of strange
undocumented shape function set that happened to need eigenvalues for something.
Lots of undocumented code, and no checks at all whether the values obtained
by eigenValuesNonSym actually make sense.

Only, for the particular choice of parameters used, eigenValuesNonSym was never
called.  Yes, you read this correctly: these hundreds of lines of code didn't
actually test anything.

The new test is copied form fieldmatrixtest.cc and adapted for DynamicMatrix
types.  It computes eigenvalues of a given 8x8 matrix and compares with the known
results.  Unfortunately, the test matrix is only symmetric.  Tests with more
general matrices are left for another day.