Fixes #93

Mostly file renaming, also removed useless #includes.

Addresses #86

Also fixed the header in Preconditioner.h

- removed wrapper functions: AllGroup, NullGroup, NullRequest
- added MPI_COMM_WORLD and other handles to MPI/DummyDefs.h
- renamed getCommunicationGroup to getCommunicator in all data structures
- improved naming to match the MPI terminology: communicator instead of group

- pointer from Grid to DistributedGrid is gone, instead the
  DistributedGrid has a localGrid attribute
- Grid has new attributes: localBegin, localEnd, interiorBegin,
  interiorEnd (useful for traversers)
- all tests work except for CutMeshFunction

Setting up a distributed grid will be solved with a mesh reader via the
function loadDistributedMesh.

Notes:

- saving and loading distributed mesh functions is commented out in
  various problems - PVTI reader/writer is needed
- disabled compilation of DistributedVectorField and DistributedGridIO
  (the tests were already broken and not executed anyway)

The Experimental directory does not make much sense - there is a lot of
experimental or just unused code in TNL, e.g. Images or Operators.

- tests moved to the main UnitTests directory
- added missing inclusion of the Algorithms tests directory to
  CMakeLists.txt

This is more suitable location because the build config tags are used
only by the GridTypeResolver and MeshTypeResolver classes.

Refactoring: renamed TypeResolver.h to resolveMeshType.h and loadDistributedMesh.h to resolveDistributedMeshType.h