ENH: allow timestamp subtraction with different timezones (GH15249)
 works on #15249 
passes the relevant tests but fails on few. Those include numpy dependency  libgfortran and some environment issues. The output works just fine.
@jreback 
If we allow this, we should also fix related ``Index`` and ``Block`` ops. @Vutsuak16 you apparently changed all of the whitespace, so you have a massive diff. Further the most important thing here is to nail down the tests to figure out what is changing. can you update closing this as stale, but if you'd like to add more tests / update pls commentt.