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.

Changed title: Feature/identity add n m → IdentityMatrix: Add methods N() and M()

mentioned in merge request !52 (merged)

mentioned in issue #4

An objections? Or is there the strong wish to remove IdentityMatrix instead? If nobody speaks up, I'll merge this.

I'd still prefer removing IdentityMatrix altogether.

@oliver.sander Would you please drum up the necessary support for the removal of IdentityMatrix and go through with it then? (e.g. at least deprecate it?)

This was made obsolete by !136 (merged).

Status changed to closed