Specialize FieldTraits for MultiIndex and LFETensor

Summary

This MR adds a specialization of the FieldTraits template for classes in dune-localfunctions that define a field_Type, namely LFETensor, Derivative and MultiIndex

mentioned in merge request dune-common!1040

As these classes do not really represent fields, we should at least document the new traits and mention that this is an intermediate solution. In general it would be better to have really a tensor, which can then decay to a vector (or matrix), if it is rank 1 (or 2).

The classes MultiIndex, LFETensor, and Derivative define already the field_type member. I have not looked in the details, but these classes call this "field", the FieldTraits specialization makes sense for these types.

This MR was mainly intended to test the corresponding branch in dune-common.

I'm not sure whether this code is ever fixed in a proper way. No one really wants to touch it.

mentioned in merge request !271 (closed)

changed title from WIP: Specialize FieldTraits for MultiIndex and LFETensor to Specialize FieldTraits for MultiIndex and LFETensor

changed the description

mentioned in commit 08e1de47

merged