WIP : Added functions to restrict forward to stc or label

These functions restrict a forward operator to specific vertices defined in an stc (e.g. after running MxNE) or from a list of labels.

The 'use_tris' and 'nuse_tris' values are not set, as triangulation with the restricted source spaces might be hard or even impossible if sources in stc or labels are sparse.

Comments, critics and hints are welcome.

Bye Daniel