Add a BaseSensor class

[ezio-melotti]

This is a WIP preview of the work I'm doing:

• [x] Create a base class for the sensor
• [x] Add tests for the base class
• [x] Create a separate class to handle the socketio connection
• [x] Add tests for the socketio class
• [x] Move the MockSensor to its own file
• [x] Use the base class for the SCD30 Sensor

Once this is done, I'll merge the PR and then create separate PRs for the items listed in #1

[GregRRoss]

I have tested the latest version of this script on my Ubuntu laptop with my SCD-30 and can confirm that it still functions for me.