MatrixObj: Add `CompatibleMatrixFilter` ?

[fingolfin]

We have CompatibleVectorFilter. I am somewhat surprised we don't also have CompatibleMatrixFilter. I don't have a need for that right now, so perhaps we hold off with work on this until we have a concrete use case, or until have adapted enough code for MatrixObj to be reasonably sure it's not necessary. But I wanted to record this asymmetry here, in case someone else (e.g. me in a year ;-) ) has the same question.