Names of derived units

[puigru]

This is a revisit of #271.
Many units are without their names because they're derived from others. This greatly reduces the usefulness of Unit#getName.

These all return null:
Units.GRAM.getName() (expected: Gram)
Units.METRE.prefix(MetricPrefix.KILO).getName() (expected: Kilometre)
Units.CUBIC_METRE.getName() (expected: Cubic metre)
... and so on and so forth.

[keilw]

Except for Units.METRE.prefix(MetricPrefix.KILO).getName() (there's an operation involved and it therefore won't even either name or symbol) the other two can get a name inside the Units collection.

[keilw]

Now all units defined inside Units got a name.