Plugins can import most commonly used FlexMeasures classes from a central place

[Flix6x]

Expose our most commonly used classes in a more central place, like: from flexmeasures import Sensor, similar to how we do it in https://github.com/SeitaBV/timely-beliefs/blob/main/timely_beliefs/__init__.py. We can then recommend plugins to import classes like that, and give ourselves some more flexibility in refactoring the location of each class (or even its name, which may be useful for renaming the GenericAsset class later).

Originally posted by @Flix6x in #343 (comment)

What are the crucial models?

• Sensor
• Asset (-> GenericAsset)
• DataSource
• User
• Account

Originally posted by @nhoening in #343 (comment)

I'd add AccountRole to that list, for service plugins.