Grids todo list

Continuing #52 (closed)...

• use .vti files for the storage of grids, drop TNLReader
• getEntityIndex() should be removed from grid - users should call entity.getIndex()
• getEntity() and getEntitiesCount() should have int Dimension template parameter
• isBoundaryEntity() should be moved from GridEntity to Grid - it is not only an entity attribute, it is always bound to the particular mesh. Generally, entities might be shared between multiple submeshes, so the method does not make sense in the general interface. There might also be read-only views for partitions of the mesh (see vienna-grid).
• [ ] the getMesh() method should be removed from GridEntity - for the same reason as isBoundaryEntity() (it is also an optimization because the size of the entity structure will be smaller)
• getCenter() and getMeasure() should be plain functions taking a Mesh and MeshEntity as parameters. This is because general MeshEntity stores only indices of the subvertices, the points have to be accessed via Mesh class. See also Effective C++ Item 23 Prefer non-member non-friend functions to member functions and the Open-closed principle.