Rename UGenOperable methods

[josephine-wolf-oberholtzer]

Let's simplify the UGenOperable method names, bringing them closer inline with Python's own math module and (where appropriate) with sclang.

Name changes:

absolute_difference -> absdiff
as_int              -> digit_value
as_maximum          -> max
as_minimum          -> min
ceiling             -> __ceil__
exponential_range   -> scale
floor               -> __floor__
power               -> __pow__
range               -> scale 
square_root         -> sqrt