Add support for dynamic importing of sensor implementations (sensor registry)

[yatharthranjan]

This would enable keeping a separate registry of sensor implementations and does not need to package in this repo ( as it can grow quite large).
Then the sensor module is loaded at runtime when and where needed based on the configuration provided. We can keep these sensor implementations in a separate repo on github (similar to how we do with questionnaire definitions for the aRMT app).

This would require an abstraction to be added to the loading of Sensors, called SensorLoader, then the default implementation could be LocalSensorLoader which will represent the current behaviour of loading through the local filesystem and another can be added RemoteHttpSensorLoader which will fetch sensor module from HTTP URL. For the latter, the httpimport library can be used. The implementations can then be used here based on what is configured for each sensor.

Another option is to publish the sensor implementations as a python package, install via requirements file and import normally like a module (it does mean that the install (and docker image) size can be large as all senor definitions will be packaged, even the ones that are not used)