Added Chroma 62000P series DC Power Supply driver

This PR is adding support for Chroma 62000P series DC Power Supplies (http://www.chromaate.com/product/62000P_series_Programmable_DC_Power_Supply.htm)

The driver currently supports:
current_limit
output_enabled
output_voltage_level
output_ovp_limit
output_measure (current and voltage measurements)
output_slew_rate

An example showing the features added can be seen here: https://gist.github.com/ceeeeej/2ab6877c03119f45c3ca

The docstrings and comments can be edited to match the coding style of the rest of the project (let me know).