Simplification of pyaeroval using Pydantic

[lewisblake]

Describe the solution you would like to see
aeroval should become it's own package in it's migration to it's own repository. This is a simplification of the code that should happen as a first step.

• Would you be able to work on this solution yourself?
  Yes. Will consult with @dulte about Pydantic.
• How can the Pyaerocom development team assist you?
  N/A.

Additional context
First step of moving aeroval out of pyaerocom