Move all pubsub topics and sensor types to constants

[Willmish]

In order to make Backend-frontend integration easier and smoother, move all string names of pubsub topics and snesor types to a single Constants.py file or Enums.

Possible solution:

• Add a constants.py file in src/tools with those constants/create Enums for each (see src/tools/status.py for an example)
• Move pubsub topics there
• Move Sensor types there
• Move Actuator types there
• Move database manager names there? or remove them entirely
• Maybe add default parameters there as well, to be stored (e.g. current pid values, selected data streamer (local SQL db/Azure DB/IoT Hub)

Possible storage files:

• Python Enums
• .env file
• .ini config file and python configparser (May be the best option, for future integration, other system settings can be stored here!)