Added support for HP6652A single-channel power supply.

I added support for the HP6652A single-channel power supply.  According to the manual, it's entirely possible that the properties I implemented will work out of the box for any HP66XXA power supply, where XX is in the 40s, 50s, 70s, 80s, or 90s.  I only tested it with the one model, so that's what I'm naming it.

Because it's only a single channel, the HP6652a class inherits from both the PowerSupply and PowerSupplyChannel base classes.

I don't have many doctests at the moment, but the ones I have pass and the text from the doctest appears on the front-panel LCD.

I based my work in part on Steven Casagrande's HP6624a class.