If the Quantity.equal method indicates that the units of the supplied
Quantity instances are not comparable by returning null, try to
convert the value of the "right" Quantity to the unit of the "left"
Quantity using the UCUM service.

And similarly for the Quantity.equivalent method with the difference
that the Equivalent evaluator returns false if the units are not
comparable.

A new method Ratio.fullEquivalent accepts a State and passes it to
EquivalentEvaluator.equivalent so that unit conversion can be used.

AddEvaluator and SubtractEvaluator, if necessary, use the
UnitConversionHelper to convert the unit of one of their
operands. This strategy requires the UcumService to be accessible in
the two evaluators which in turn means that any evaluator which
performs addition or subtraction now requires the State instance.

A few tests which had to be skipped before this change succeed
now. However, the tests
QtyIvlCollapse_CollapseQuantityUnits[Not]WithinPer now have to be
skipped until comparison operators for Quantity consider unit
conversion.

* {Less:,LessOrEqual,Greater,GreaterOrEqual}Evaluator use the new
  helper function compareQuantities to support comparison of Quantities
  with different but comparable units

* CqlList uses the new comparison behavior when sorting, and therefore
  needs the current State instance

* Interval.get{Start,End} for the unbounded side of an interval return
  a Quantity with the correct unit (the unit of the opposite end point)

* Enable previously skipped tests

Add implementations in createUcumService and DefaultUcumService.

I think this is because Interval with Quantity end points of different
units now work but are expected not work.

* MultiplyEvaluator and DivideEvaluator, if necessary, use the
  UcumService to compute the value and unit of the operation result

* The various Variance and StdDev evaluators now shared code for the
  variance computation. The variance computation can produce a
  quantity with either a squared or a non-squared unit so that the
  both variances and standard deviations use to correct unit.

* Enable previously skipped tests which involve the above evaluators
  and unit conversion