add Sensor class with clear data model

[earth-chris]

Data collection / sensor management data were handled in a very strange fashion:

• a python script, create_collection.py, was used to define the collections in a python dictionary.
• this script would write a JSON file to earthlib/data/collections.json
• the json file would be read and returned as a dictionary

This is all very silly. This refactor creates a dataclass object that defines the key properties we need to track for the different earth observing sensors we support. This keeps everything in python, and does not require tracking and including data files.

This MR deletes the collections.json file, the script for generating it, the make target, and simplifies some patterns around getting key pairs.