ENH: Expanding functionality for mixed source spaces and source estimates

Things like this make me think comments are critical here... You used to have to have this line of code here, presumably because you needed it. However, now you're deleting it, presumably because you realized it isn't necessary (or maybe the `.T` above makes it not necessary). But why? What's going on here?

Actually, instead of using `np.dot`, you should really be using `combine_transforms` (or whatever it's called) and `apply_transform` from `transforms.py`. These functions ensure that your `to` and `from` of the dot product match, and (in the case of `combine_transforms`) return transforms with appropriately populated `to` and `from` parameters itself. This forces you to code with explicit transform type matching. That, combined with comments, would greatly clarify these parts of the code. Make sense?