ENH: Adding STC expansion

This basically allows one to un-do `stc.in_label()` operations. Can be useful in simulation, I'm finding.

Note that I added a check on `__add__()` and `__iadd__()` that the `vertno` from each `SourceEstimate` must match. We could basically use this type of functionality to expand the STCs if they don't match, but I think it's much safer to make the user expand each STC first, if they want, then add them together.