meataxe: extend invariant form detection

[fingolfin]

TODO: complete it, add tests

[hulpke]

A main change seems a change from IsomorphismIrred to IsomorphismModules. What I want to make sure is that there will not be a chance of running in infinite recursion.

I have not looked again into the code of these two functions, but the former is a straightforward spinning algorithm, while the second is a rather elaborate calculation that uses a lot of other MeatAxe routines.
This means that with this change, InvariantXXXForm cannot be used within routines that might get called from IsomorphismModules. This means that the tests need to cover a variety of module structures (direct sums of multiple isomorphic and non-isomorphic indecomposable components).