Clarify unit "cup"

[mkllnk]

taxonomies/measures.rdf

Lines 1013 to 1021 in f562389

	<rdf:Description rdf:about="https://github.com/datafoodconsortium/taxonomies/releases/latest/download/measures.rdf#Cup">
	<rdf:type rdf:resource="http://www.w3.org/2004/02/skos/core#Concept"/>
	<skos:example xml:lang="en">1 handful = 0.5 cup</skos:example>
	<skos:inScheme rdf:resource="https://github.com/datafoodconsortium/taxonomies/releases/latest/download/measures.rdf#DFC_ProductGlossary_Measure"/>
	<skos:prefLabel xml:lang="en">cup</skos:prefLabel>
	<skos:prefLabel xml:lang="fr">tasse</skos:prefLabel>
	<skos:notation>cu</skos:notation>
	<skos:broader rdf:resource="https://github.com/datafoodconsortium/taxonomies/releases/latest/download/measures.rdf#QuantityUnit"/>
	</rdf:Description>

I hadn't seen the definition of a cup being two handfuls so far. A handful seems to be an approximate measure as well. Unfortunately, there are many definitions of a cup, ranging somewhere between 200ml (Japan) and 250ml (Metric).

With my European upbringing, living in Australia, I would choose the Metric cup but maybe we actually need to define several cups. The US Legal cup of 240 ml is a strong candidate. The others are probably not used enough to include them.

I also noticed that some concepts refer to a unit: but I couldn't find that in the context. For example:
https://github.com/datafoodconsortium/taxonomies/blob/f562389edd307ed5ffda3b6a55ed208ce679dfc6/measures.rdf#L142C2-L142C45