UCUM grammar changing to require exponents be pushed down to base names

[dmcclean]

The UCUM are changing their grammar to disallow exponents applied to grouped products of unit names, and to instead accept them only on base names. This will require a change to the interchange name machinery which maps our UnitNames to UCUM names.

See http://unitsofmeasure.org/ucum.html#para-10 for details.

Up until revision 1.9 there was a third clause “Since a unit term in parenthesis can be used in place of a simple unit, an exponent may follow on a closing parenthesis which raises the whole term within the parentheses to the power.” However this feature was inconsistent with any BNF or other syntax description ever provided, was never used and seems to have no relevant use case. For this reason this clause has been stricken. This is a tentative change. Users who have used this feature in the past, should please comment on this deprecation. If we receive indication that this feature was used by anyone, we would undo the deprecation. If no comments are received, the deprecation continues to take effect.