Improvements to FindPetsc.cmake module

Hi,

building opm-core with the petsc patches revealed some problems in the FindPetsc.cmake file - some details seemed very specific to your system. Please merge these patches.
I'll have a look at it.

The Ubuntu petsc package does -not- include mpiuni, and the makefile should really reflect this, using mpiuni only as a fallback. I'll write up a patch right away.