Add XTCE UnitSet support when reading from spreadsheet

The XTCE standard allows zero or more UnitType instantiations,
combined into one UnitSet.

In the SpreadsheetLoader, we only support one UnitType
(as encoded in the "eng unit" column of the Parameters sheet)
Merge pull request #16 from fqqb/unit-set

Add XTCE UnitSet support when reading from spreadsheet