Validation preprocessor

[MayerBjoern]

This pull request is for a preliminary review. The code is tested, but i have not yet created pytest test cases or updated the docs.

Features:

• Allowing partial object updates if the new config variable ALLOW_PARTIAL_OBJECT_UPDATES is true
• Automatically assume that the unit of a Quantity matches the scheme unit if not given
• Accept either magnitude or magnitude_in_base_units or both for a Quantity object. If both are given they have to match each other, otherwise automatically calculate the other one
• Return both magnitude and magnitude_in_base_units

Additional fixes:

• Boolean config variables from the env were represented as a string despite the default value being set as an actual boolean
• The quantity validation function created a new Quantity object based on the magnitude_in_base_units with the non-base unit despite the old version of the constructor only expecting an already converted magnitude

[FlorianRhiem]

Thank you for the pull request!

I think they way you integrated the preprocessor should be fine, as no copies are being validated right now, though this needs to be documented to make sure nobody valdidates (and thereby implicitly fixes) a copy of some invalid data. Maybe the modifications done by the preprocessor should be enabled by a parameter (False by default) to avoid accidents like that?

Partial updates via the API can definitely be useful to make life easier, though I think it would be better to have this as an explicit decision by the user, instead of it being based on configuration value. Perhaps by using the PATCH verb instead of POST? PATCH might be more meant for in-place changes instead of creating a new resource as it's done here. If it was done this way instead of changing the established behavior, the configuration variable might not be necessary.

You perform an equality check between the pint quantities. Is there a guarantee that float arithmetic cannot cause a false inequality? If not, a small relative epsilon might be better?

If it's not too much work for you, maybe split this PR into two, one for the quantity changes (and the general preprocessor setup), and one for the partial updates built on top of that one?