Merge: Definition of "searchVolumeInProductOfUnits"

**Term**: `searchVolumeInProductOfUnits`
**Definition**: "Total number of product of coordinate units within the search volume".
**Original comment**: [link](https://docs.google.com/spreadsheets/d/16pC2cDsdxlzv2CzlNMtStqt5-xHwDEsU6MjZVxWhrE4/edit?disco=AAAAAGsZF9o)
**Current link**:  [nidm-results.owl/searchVolumeInProductOfUnits](https://github.com/incf-nidash/nidm/blob/master/nidm/nidm-results/nidm-results.owl#L1001-L1012)

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@khelm `19:46 9 Apr`
I'm not sure what this is going for "of product of coordinate..."?
	
@cmaumet `08:28 24 Apr`
Examples of "product of coordinate units" would be mm^3 (if units is [mm, mm, mm]) or mm*s (if units is [mm, s]). Does that make sense?