Avoid names which differ only by T

[chiphogg]

We strive to be "easy to use correctly; hard to use incorrectly".

Right now, one significant way the library is "easy to use incorrectly" is to do with very different type names that differ only by T. Examples:

• UnitProductT and UnitProduct.
  • The former accepts any units in any order. The latter has strict rules for ordering and should only ever be auto-generated.
• CommonUnitT and CommonUnit
  • Same considerations as above.

For UnitProduct, I could see using something like CompoundUnit. I don't have any good ideas for an alternative name for CommonUnit.