Fix for PETSc 3.12.4 (#304)

Description:
For this newer version of PETSc, a parallel run using SuperLU_dist needs to
specifically call that solver (not SuperLU, which only works for serial).  Older
versions of PETSc allowed the solver to be set to SuperLU whether it was serial
or parallel.

CASL Ticket # - 4661

src/LinearSolverTypes.f90

@@ -546,7 +546,14 @@ SUBROUTINE init_LinearSolverType_Base(solver,Params,A)
           !PC calls
           CALL KSPGetPC(solver%ksp,pc,ierr)
           CALL PCSetType(pc,PCLU,iperr)
-#if (((PETSC_VERSION_MAJOR>=3) && (PETSC_VERSION_MINOR>6)) || (PETSC_VERSION_MAJOR>=4))
+#if   (((PETSC_VERSION_MAJOR>=3) && (PETSC_VERSION_MINOR>=12)) || (PETSC_VERSION_MAJOR>=4))
+          IF (solver%MPIparallelEnv%nproc>1) THEN
+             CALL PCFactorSetMatSolverType(pc,MATSOLVERSUPERLU_DIST,iperr)
+          ELSE
+             CALL PCFactorSetMatSolverType(pc,MATSOLVERSUPERLU,iperr)
+          ENDIF
+          CALL PCFactorSetUpMatSolverType(pc,iperr)
+#elif (PETSC_VERSION_MAJOR>=3) && (PETSC_VERSION_MINOR>6)
           CALL PCFactorSetMatSolverType(pc,MATSOLVERSUPERLU,iperr)
           CALL PCFactorSetUpMatSolverType(pc,iperr)
 #else