Draft: Implemented satellite for a Keithley6517B

Controlling a Keithley power supply over USB-to-RS232 adapter and pySerial. It works, even if some elegance may be missing. To do:

• Document
• Make sure the naming and file placement is up to convention (each satellite in its own directory would be nice I think, but I'm not sure how to do it in a Python-friendly way)
• The custom commands time out, which is not so nice (see #97 (closed))
• Also get it going for a 2410 (should be relatively easy as the more tricky parts of the code already exist)