Use system-provided MUMPS library for AGMG

This require us to compile in a small glue module which maps the
name of the internal routine to the system routine.

cmake/Templates/dagmg_mumps.f90

@@ -0,0 +1,11 @@
+!     AGMG expects an interface to MUMPS named like this
+      subroutine DAGMG_MUMPS (id)
+
+!       In here are the official defines for the parameter passed to
+!       MUMPS. These must match the corresponding ones in dagmg.f90
+        include 'dmumps_struc.h'
+        type (DMUMPS_STRUC) :: id
+
+!       Let the system library do all the heavy lifting
+        call DMUMPS (id)
+      end subroutine DAGMG_MUMPS