Support xtce:UnitSet

[medley56]

XTCE allows for compound units by specifying multiple <xtce:Unit> elements for a ParameterType. This is discussed in section 4.3.2.2.4 of the XTCE element description green book, here: https://public.ccsds.org/Pubs/660x1g2.pdf (p. 65).

A compound unit will have to be represented in a ParsedDataItem. This could be solved by making the unit attribute of ParsedDataItem either a string or an iterable of strings (e.g. a tuple or list) to support compound units.