Provide actor access to the FPGA CALIB command

One of the FPGA commands is to scan through a range of driving frequencies for the piezos while measuring current, in order to find the peak resonance. Add an actor command for that.

OK, I have:

• fps calib [@ccw] [@updateModel]
• fps writeModel <xml>

The existing FPGA commands are all simple names (reset/power/hk/diag), so I stuck with that. And the commonly used loadModel command takes an <xml> argument so I stuck with that usage.
Not loving either. Also, might make updateModel the default. There is no obvious value in not doing that.