Resurrect col_unit elements, especially for optional columns

[nilshoffmann]

In PR #124, colUnits were removed from the specification. However, for optional columns, there is currently no simple way to define which unit the values they report hold, if their column name is not specified via a CVParam. AFAIU, user params do not make sense for optional column names, thus, an optional column with a specified name can only have assigned units for the values it reports via the col_unit mechanism.

[andrewrobertjones]

So you mean have something like the following:

MTD colunit-small molecule opt_ms_run[1]_cv_MS:MS:1002476_ion_mobility_drift_time=[UO,UO:0000031, minute,]

Yes I agree that make sense. Shall I re-instate it?

[nilshoffmann]

Yes, please. The code is still working :-)

[nilshoffmann]

Or for the opt_global_mass_error column: MTD colunit-small_molecule_evidence opt_global_mass_error=[UO, UO:0000169, parts per million, ]

[andrewrobertjones]

Re-instated but needs a new CV term for my example to be valid, which I posted a request for

[nilshoffmann]

Fixed by #135