do use eigen's umfpack support anymore

use BiCGStab instead. it might be slower, but at least it is always
available...
Merge remote-tracking branch 'andlaus/no-umfpack' into optional-umfpack
Use UMFPACK solver if available in sim_simple.cpp.

Fall back to Eigen's BiCGSTAB solver if UMFPACK is not available.  The
BiCGSTAB is built into Eigen and consequently always available when
Eigen is found.
Merge pull request #17 from bska/optional-umfpack

Use UMPACK only if available