feature: create ConfigurableParameter() class

[ba58smith]

From a Slack conversation:
jkoz [1:41 PM]
Re: configurable value: Matti - you have this "ObservableValue" object you used for the individual GPS values (and some other things IIRC). That concept - have the value wrapped by a class, is very close to an idea I had: We could make a "ConfigurableParameter" class that is much like ObservableValue, but it could also supply itself as json, its own title etc. If we did that, then we could probably get away with just declaring the values you want to use in a Transform as a "ConfigurableParameter", and then all the other code (get_configuration(), set_configuration(), get_json_schema()) would work with that. That would prevent having to write the marshalling code over and over again.