Improvements to composition mechanisms

[esoubies]

Some examples of stuff that have to be improved:

• In Map plus_ and minus_ test if equal to itself (or a scaled version of itself) and return the corresponding scaled version of itself. Do the analogy in LinOp and Cost.
• In Map makeComposition_ test if inverse and return identity (the analogy of makeComposition_ in MapInversion)
• For LinOpConv if we do H^-1*H we get a LinOpConv and not an identity. In makeComposition_ of LinOpConv test if the new mtf is constant and return the appropriate LinOpDiag. This is more general and holds other LinOps that have a new implementation of mpower_
• add the possibility to add a scalar to a Map for adding a scaled identity