simplify "segmentations" command keyword options

[Elaine Meng]

(1) Consolidate resetMouseModes + setMouseModes into one keyword option with true/false or on/off values (whichever you prefer), keyword could be setMouseModes or even mouseModes

(2) Consolidate resetHandModes + setHandModes into one keyword option with true/false or on/off values, keyword could be setHandModes or even handModes

(3) consolidate sphereCenter + (planeCenter + slice) into one keyword option with 3-tuple value, keyword of your choice, could even be center. Whether 2D or 3D is indicated by whether or not an axis is given. If axis is given, then it's 2D.

[Zach Pearson]

Partially fixed by ​this commit (parts 1 and 2).

[Zach Pearson]

​This one fixes number three. I've also added a command line argument to the core -- OnOffArg -- that prints "on or off" in usage strings instead of "true or false", when arguments such as "handModes" make more sense as "handModes on|off" instead of "handModes true|false".

[Zach Pearson]

re|setMouseModes moved to mouseModes
re|setHandModes moved to handModes
as discussed, center is now one keyword argument that takes three integers. if an axis is specified, the third value is taken as the slice