Validation of volume/weight value for Sample Type's Minimum Volume

[xispa]

Description of the issue/feature this PR addresses

Requires https://github.com/senaite/senaite.core/pull/1768

This Pull Request adds a validator for the MinimumVolume field of Sample Type content type to ensure the value set is a valid volume or weight physical quantity

-- I confirm I have tested this PR thoroughly and coded it according to PEP8 and Plone's Python styleguide standards.

[toropok]

IMHO, I dont think this field should be required at all or we need to think on more universal and general approach. We use Senaite in 3 genetics labs and they dont have any measurable in volume/weight metric incoming samples. This field is not optional and we put meaningless text there like: "cotton swab" :) In genetics they get something measurable after sample preparation step, when DNA extracted and solution prepared.