Make PMGSolve() to obey 'MG Lowest Linear Solver' keyword.


git-svn-id: svn://svn.code.sf.net/p/elmerfem/code/trunk@6950 04224531-1f84-46d8-96ce-875b9e37dce3
Fixes to prism treatment and parallel halo.


git-svn-id: svn://svn.code.sf.net/p/elmerfem/code/trunk@6951 04224531-1f84-46d8-96ce-875b9e37dce3