overhaul units validation in bids-validator

[sappelhoff]

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we are currently in the final review round of a PR in bids-specification that will RECOMMEND CMIXF formatting of SI units: https://github.com/bids-standard/bids-specification/pull/411

Once that PR is merged, we should "switch off" units validation as it is currently and then start re-implementing / changing the way we validate to be in line with the specification again.

Specifically, units should be checked whether they conform to CMIXF-12 (+ the 5 unicodes we include for backward compatibility) ...

• if they don't --> send a warning (not an error)

outdated, because the PR changed

~We also need to consider cases where SI units are impossible -> in these cases, users can specify their units in JSON sidecars. (This needs to be covered by the tests, i.e., IF invalid SI unit found, but unit specified in JSON, do not raise ... else send error "please format as CMIXF")~

~Lastly, the validator should alert users when the INVALID unicode U+03BC (μ) is used instead of the VALID U+00B5 (µ). The warning should include something like "please take extra care ... these characters look the same, but are different"~