Fix petsc master build:
Fix petsc master build:

Attach nullspace to Mat instead of KSP to restore build with petsc-master, replacing KSPGet/SetNullSpace with MatGet/SetNullSpace.

Tested with petsc master by @drhodrid

rjferrier/fluidity,iakovos-panourgias/fluidity,jrper/fluidity,jjo31/ATHAM-Fluidity,jjo31/ATHAM-Fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,rjferrier/fluidity,rjferrier/fluidity,iakovos-panourgias/fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,rjferrier/fluidity,rjferrier/fluidity,jrper/fluidity,iakovos-panourgias/fluidity,jrper/fluidity,iakovos-panourgias/fluidity,jjo31/ATHAM-Fluidity,iakovos-panourgias/fluidity