Added heatmap plotting method to Alignment (and tests).

I think you might have branched before the old `UserWarning`s were replaced by `DeprecationWarning`, but you're reintroducing them. Since you only have one commit, I'd probably recommend creating a new branch fresh from master, and cutting/pasting the new heatmap method and test method there. That way, you won't have to hunt for every DeprecationWarning that you're unintentionally modifying here and change them back. Thanks!