IdentityMatrix: Add methods N() and M()

Since it's very difficult to make progress on !52 (merged), I'll start to break it up into smaller, less controversial changes. This is the second piece that I'd also like to get out of the way. The feature is essentially uncontroversial: matrices can be asked for their dimensions through methods N() and M(). Instances of the IdentityMatrix class cannot for some reason. Now they can.

The only good reason against merging this is that I've repeatedly heard the IdentityMatrix should be abandoned altogether. If there's a consensus for that, I'll happily come up with a merge request to deprecate this class instead.