Fix problems when parsing plural units with a prefix.

• In the unit_kw grammar object, test for a prior match properly when looking for prefixes by n-gram (it was testing whether a unit had been detected).
• When a match is found in System::evaluateUnitPrefixToken(), stop iterating so the iterator is left in a valid state.
• Prefixes should never have a plural form.
• Add a Unit::multiplier() method for regression testing purposes.
• Add regression tests.
• Add a feature to the unit-converter utility program: when passed a measurement and unit on the command line, perform the conversion as specified and exit rather than processing user input.