Refactor Quantity into a sub-directory

[FlorianDeconinck]

We need to write a system that allow for the manipulation of Bundle of data (Tracers, Aerosols...). We hope to use the already establish Quantity to do so. This PR aims at cleaning up the quantity.py, which was a single file mixing at least 3 different concepts. A subsequent PR will propose a "Bundle" as an extension to Quantity

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included

[fmalatino]

like this a lot. lgtm

[oelbert]

This makes a lot of sense to me