Some of the Measurement related property names should be corrected

[goksel]

The following changes should be made to be consistent with the Ontology of Units of Measure:

• om:alternativeSymbols [0..] --> om:alternativeSymbol [0..]
• om:label[1] --> rdfs:label[1]
• om:alternativeLabels [0..] --> om:alternativeLabel [0..]
• om:comment[1] --> rdfs:comment[1]

rdfs points to "http://www.w3.org/2000/01/rdf-schema#"

[IsabelMarleen]

Pull request #407 solves this issue