Update standard filename setter for external engines in OPS 2.0

[sroet]

This does a couple things:

• cherry-picks in Add FutureWarning for default filesetter external engines #1102
• the default filename_setter for ExternalEngines are now RandomStringFilenames instead of FilenameSetter
• FilenameSetter is now a subclass of RandomStringFilenames (instead of the other way around) (this was needed to still be able to use Filename setter after changing the default options)
• FilenameSetter now returns a string that is padded up to self.length` with zeroes
• GromacsEngine.set_filenames only resets the filename_setter if the filename_setter has a reset attribute
• updated the tests for this behaviout
• solved pep8 complaints on all touched files
• dropped nose from all touched tests

part of the discussion on #1101

[sroet]

failing CI is due know issue with importing openmm (or openmmtools) in notebooks is raising a warning:

Traceback:
Unexpected output fields from running code: {'stderr'}

[sroet]

@dwhswenson all green, this is ready for review