LilSparseMatrix::swap() assumes matrices are the same size

[mcmillan03]

Need to either restrict swap to matrices that have the same number of rows and columns or handle different sizes.

[MaiaBlanco]

Is this a question of API or what we need in the back end?

From a user's perspective, I think it only makes sense to swap two matrices if they are "congruent" in some way - namely the dimensions. Otherwise you wouldn't be able to rely on the swapped matrix remaining compatible with later operations (e.g. MxV) in the way the original un-swapped matrix would have been.