instead, blacs context has to be set beforehand

if not using Hermitian multiply

Added C interface for generalized eigen problem
Extended the functionality of test.c to check generalized eigenproblem,
both with and without already decomposed matrix

added possibility to re-use the already decomposed matrix B in the
generalized problem
Added respective test

* tested with elpa1 only (should be sufficient)
* not using Hermitian multiply at the moment
* requires scalapack

and some modifications in test.f90

executables were generated event though the functionality is not present
in test.c