Add Observation base class

[cdeil]

I noticed in https://docs.gammapy.org/dev/api/gammapy.cube.MapDatasetMaker.html that all the docstrings say that they support DataStoreObservation.

Really now we have two classes that are (or should be) supported here and in other places where an observation is used:

• https://docs.gammapy.org/dev/api/gammapy.data.Observation.html
• https://docs.gammapy.org/dev/api/gammapy.data.DataStoreObservation.html

I suggest to resolve this issue by introducing a base class, like this:

class Observation:
    """Observation base class."""
    # empty, nothing for now
    # will grow methods, might become an abc.ABC

class MemoryObservation(Observation):
    # exactly what we have now

class DataStoreObservation(Observation):
    # exactly what we have now

and to adapt docstrings like in MapDatasetMaker (and isinstance checks if we have any).

That's something that can be done in ~ 1 hour, and IMO is the right, achievable solution for v1.0.
The Observation and Observations parallels the design of Dataset and Datasets, where Dataset is a small base class: https://docs.gammapy.org/dev/api/gammapy.modeling.Dataset.html

@AtreyeeS @adonath @registerrier - Thoughts?

Anyone willing to send a PR? (otherwise I'd be happy to do it, if it's agreed that it's an improvement)

[adonath]

Solved in #2753.