Add State machine to sensors

[yatharthranjan]

Adding a state machine to sensors will help in keeping track of any changes in sensors.
This will also help in reporting these to a status/<sensor-name> in the pub sub system similar to #5 .

State machine libraries for py-

• https://github.com/pytransitions/transitions
• https://pypi.org/project/python-statemachine/

Custom Implementation -

• https://python-3-patterns-idioms-test.readthedocs.io/en/latest/StateMachine.html
• https://dev.to/karn/building-a-simple-state-machine-in-python
• https://www.python-course.eu/finite_state_machine.php

EDIT- A really simple way would be using http://liftoff.github.io/onoff/

EDIT 2 - Nice tutorial using the python-statemachine library -- https://www.askpython.com/python-modules/state-machines-python

### Tasks
- [x] Read the code and understand what needs to be changed and come up with an plan for implementation. Only the python module is involved in this currently.
- [x] Add new abstract functions in the sensor class to expose the states
- [ ] Add states that a sensor can use.
- [ ] Push sensor states when a state change happens (can also use pull using functions above)
- [ ] Publish to the pub/sub broker using Publisher, similar to the way other data is published.