Deprecate periodic input ...

[jbusecke]

This is my attempt to resolve a bunch of issues that stem from the fact that we still support periodic as input. This should stop right here.

This is a reimplementation of #534 which is quite far behind at this point (my apologies to the original author for falling behind on the maintainer duties here).

Totally just taking a first stab here to give people a point of reference. This needs some careful (not past 9pm) consideration of how to change the tests. The fact that periodic is so deeply embedded into the test suite is really the reason that it was not removed earlier.

• Closes boundary="periodic"? #195,[BUG] Diff in non periodic axis seems to be periodic #509, [BUG] Default boundary condition is periodic even when specified otherwise #604, [BUG] Non-intuitive behavior if periodic not set as list #624, [BUG] cumsum seems to apply periodic B.C. even when it knows that is not the case.  #625
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst