Add mechanism to specify active/inactive cobras

We need a way to specify which cobras should be enabled for moves and which should not be moved. For example, PSF characterization will want to hide all but a set of DCB fibers, or all but every fifth fiber, etc.

One proposed mechanism: add a <maskFile> argument to the main fps motion commands (home, moveToPfsDesign, moveThetaSteps, etc). This file would have rows with GFM fiberIds and a "masked" column to not move, in addition to the already masked broken cobras. We might later use this mechanism for other things, so probably best to have a named "masked" column for this purpose.

Not obvious whether/how we would reuse these. Another ticket for later.