add optional arguments to doFocusLoop

[nlourie]

improve the functionality of the doFocusLoop command (the wintercmd AND the do_FocusLoop method of roboOperator).

• [x] arguments for describing the focus loop parameters: a centerpoint (Z0) and throw (dZ) from the command line. this will make it much easier to find the center of focus manually.
• [ ] add argument to allow multiple focus iterations (eg, n_iter = 2 means that it will do a focus loop and then repeat around the center point. may want to do a fine/coarse option here.

[nlourie]

implemented the more general method, but not the multiple try thing, seems like we can handle without it when we have a good thermal model.