Introduce a units module

[lewisblake]

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pyaerocom should have a units module where all functions related to unit declarations and conversions are stored. Ideally a target unit would be provided for each variable, and then if not available the default behavior of converting to the units of the observations could continue.

This is important for a number of reasons. Most recently it came up because super-observations where networks report the same variable in different units can not currently be created correctly.

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Related issues inlcude:

• aggregating several obs networks using aeroval fails when the networks don't provide the same unit natively #1499
• add units systematically when mean and stdev are provided #1021
• nice and unique units in pyaerocom output for aeroval #1411