Modify the MCMGMapper to store multiple dofs per geometry type

The layout function can now instead of a bool return the number of degrees of freedom to be stored on a given geometry type.

• The index method returns the starting point of the block in the dof vector
• a new method indices returns an iterable range with the indices of the given entity.

  for ( auto i : mapper.indices(element) )
     // do something with vector[i];

Note: the older layouts still work since true -> 1 and false -> 0 so that one dof will be stored per geometry type included in the layout as before.

Open question (see below) concerns deprecating methods that can now be replaced using the indices method (i.e. contains)