Add support in RDFLib for UCUM and LINDT for arithmetical operations on units of measurement

[ashleysommer]

Would be great to add this to the RDFLib SPARQL executor, as well as support in RDFLib-native Literal operations.
w3c/sparql-dev#129

[ashleysommer]

https://ucum.nlm.nih.gov/ says "UCUM has been adopted internationally by many organizations such as IEEE, DICOM, LOINC, and HL7, and is also in the ISO 11240:2012 standard". (and in addition to the JS implementation has more resources like validation and autocomplete)

[ashleysommer]

There is a python library pyucum but it appears to be very application-specific, and depends on big libraries:

    numpy>=1.15.0
    pandas>=0.23.0
    tqdm>=4.32.2
    seaborn>=0.9.0
    matplotlib>=3.1.1

So we're probably better off doing our own.