Allow packages to add source code to the project

By returning a ${package}_SOURCES variable with the names of the code,
it will be propagated to ${project}_SOURCES and subsequently compiled
into the library.

AGMG (Algebraic Multigrid) uses this functionality.
Make opm-core pick up AGMG through the standard list
Don't add AGMG in an ad-hoc way

Use the standard prerequisite list instead.
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.
Enable use of parallel version of MUMPS

If we have enabled MPI, then we must call MPI_Init and MPI_Finalize
in the program anyway so we may as well use the parallel version of
MUMPS.
Use parallel version if and only if MPI is installed
Function has a different name in the parallel vs. serial version
Merge pull request #308 from rolk/308_agmg

Resurrect support for AGMG library

This is an interim milestone that may serve as point from which to 
resurrect the AGMG suppert fully if the AGMG author ever decides to 
re-release the software under an open licence.