A FunctionQuantity is a Function that
maps from one or more instances of ConstantQuantity to another instance
of ConstantQuantity. For example, the velocity of a particle would be
represented by a FunctionQuantity mapping values of time (which are
ConstantQuantities) to values of distance (also ConstantQuantities).
Note that all instances of FunctionQuantity are Functions with a fixed
arity. Note too that all elements of the range of a FunctionQuantity
have the same physical dimension as the FunctionQuantity itself.