Add multifab method that takes an int for the direction

[dpgrote]

This adds a new way to call the multifab routine from Python, taking an integer for the direction instead of requiring an instance of the Direction class.

This also cleans up the documentation some for the other versions of the routine.

[EZoni]

@dpgrote
You might need to rebase after #5474 to fix the Clang sanitizer checks.