Fix bug in solveForQuantity:
 solveForQuantity supported solving for f( x, quantity(x)),
 however findIndVarOfMinAbsQuantity did not.  Thus if
 function was specified, and solveForQuantity failed
 to bracket the root, an incorrect recovered value from
 findIndVarOfMinAbsQuantity was returned.
 findIndVarOfMinAbsQuantity now supports taking a
 function and a target value.