Required properties in objectiveLensUsage

[Alixbonard]

Should immersionMedium be a required term ?

NB: objectiveLensUsage schema:

Field	Description	Required	Type
immersionMedium	Immersion medium between lens and sample (air, water, oil…). If chemicalMixture overkill can be controlled term	Yes (Maybe optional)	linked -> chemicalMixture
correctionCollar	Setting of the objective lens correction collar, used to compensate for coverslip thickness or spherical aberrations. Typically given in millimeters.	Optional	QuantitativeValue