InSitu tests are always build now.
VtkMappedMesh should be complete.

Conflicts:
	CMakeLists.txt
	Tests/CMakeLists.txt
Initial implementation of VtkMappedMeshSource.
VtkMappedMeshSource is working.

Remaining todo: point and cell data.
Some comments and cleanup.
Added a mesh conversion round trip test.

It takes an OGS mesh, converts it via VtkMappedMeshSource to VTK, then
the VTK mesh gets converted back to OGS mesh.