Improve performance when requesting contours in combined files

[thorbjoernl]

Change Summary

• Removes support for asyncio (wasn't being used, and low performance; see Pyaerocom tests fail with SystemError: (11, 'Resource temporarily unavailable') in tests #46 (comment))
• Caches all timesteps in file separately when initially reading a contour file, so iterating over timesteps is more performant.
• Iterating over the largest contour file (~140MB) in IFS takes ~24s compared to 16s when requesting the entire file in one go.

Related issue number

closes #110

Checklist

• Start with a draft-PR
• The PR title is a good summary of the changes
• PR is set to AeroTools and a tentative milestone
• Documentation reflects the changes where applicable
• Tests for the changes exist where applicable
• Tests pass locally
• Tests pass on CI
• At least 1 reviewer is selected
• Make PR ready to review

[heikoklein]

Looks good, but please consider renaming some functions/classes to more commonly used ones, and remove strange # @override comments

[heikoklein]

Nice, thanks. Just some smaller comments.

[heikoklein]

rename utils/typing.py to compat/typing.py since it is only a compatibility layer.