File powersupply.py defines the interface to be used by child classes
implementing it. It also provides helpers to allocate a child class
based on configuration provided ('type' field).
File powersupply_sispm.py is an implementation using pysispm , as it's
the one used to control the programmable power socket we have right now.
This kind of class will be used in later commits by Nanobts class, as we
want to poweroff the Nanobts completelly when not in use.
Using it requires the following extra dependencies:
$ apt-get install python3-usb
$ pip3 install pysispm