Keithley: port to pyvisa and send metric in each step of ramp

This ports the Keithley satellite to pyvisa, improves the configuration reading (related to #320 (closed)), and moves the ramp function to the satellites to prepare for sending the voltage out after each step once !1012 (merged) is merged.

Remaining TODOs:

• Attempt to ramp down on failure (#320 (closed))
• Send voltage metrics after each ramp step once !1012 (merged) is merged
• Test with hardware

Requires !1012 (merged)