[FIX] Rename "Unit" metadata to "Units" for consistency with existing fields

[effigies]

As noted in #622 (comment), we previously had in anatomy imaging data:

bids-specification/src/04-modality-specific-files/01-magnetic-resonance-imaging-data.md

Lines 222 to 233 in a7faded

	Phase images MAY be in radians or in arbitrary units.
	The sidecar JSON file MUST include the units of the `phase` image.
	The possible options are `rad` or `arbitrary`.
	For example:
	sub-01_part-phase_T1w.json
	```Text
	{
	"Units": "rad"
	}
	```

Given that this metadata field has the same intent, to describe the interpretation of voxel values in an imaging file, we should be consistent.

@mnoergaard @melanieganz I do not know if this means we should also change, e.g., InjectedMassUnit.

@tsalo I notice that we don't seem to have this field in a metadata table anywhere. Do you have a suggestion of where to add it?

[tsalo]

@tsalo I notice that we don't seem to have this field in a metadata table anywhere. Do you have a suggestion of where to add it?

In many cases, suffixes are restricted to one specific unit (usually "arbitrary"). I'll need to dig around to see which suffixes (or entity-value combinations) allow "Units".

[sappelhoff]

@mnoergaard @melanieganz I do not know if this means we should also change, e.g., InjectedMassUnit.

that'd sound better (more consistent) to me.

[mnoergaard]

I am also fine with this (for all names including units). I will go ahead and update the example data sets and the validator.

validator (using code suggestions): bids-standard/bids-validator@866049d (and following commits)
examples : bids-standard/bids-examples#269

[effigies]

Full list of changed fields:

• Units
• InjectedRadioactivityUnits
• InjectedMassUnits
• SpecificRadioactivityUnits
• TracerMolecularWeightUnits
• InjectedMassPerWeightUnits
• MolarActivityUnits
• InfusionSpeedUnits
• PharmaceuticalDoseUnits
• ReconMethodParameterUnits

[effigies]

Can we get a second review?