Address issparse for specialized matrix types part of #13096

[pkofod]

Adds issparse methods for specialized matrix types, such that it returns true if the matrix they are based on are sparse.

cc @kshyatt

[tkelman]

Looks reasonable. Could you rebase and squash the commits?

[pkofod]

Yes, although I seem to have done something wrong :)

[tkelman]

Yeah, this shouldn't have the other commit from a different PR in it. If you rebase again but delete the line for that other commit it should fix it.

[pkofod]

Sigh, I always mess this rebase-thing up. I need to find a git course :)

When you say "rebase again but delete the line for that other commit" what do you mean by that ? (by the way I seem to have added another commit now.

Edit: I seem to have figured it out :)