Feature/manual indextree

Until know the text only described special constructions for multi-indices that are derived from the FE tree. However, this is not really what the interface does because we support more general things. For example even a simple basis may already provide non-trivial multi-indices (e.g. for DG with element blocking).

This MR adds a description of the general concept for multi-indices and prefix-size that is characteristic to our interface. The old text describing the special constructions should IMO be moved to some place where we describe the construction of bases using Power* and Composite*.

This is a follow up to !45 (merged) .