Dimensionless quantities have all exponents equal to zero.
Such quantities are simply numbers. The associated unit is
the unit one, symbol 1, although this is rarely explicitly
written.
See chapter 2.3.3 Dimensions of quantities, The International
System of Units, 9th edition.
Quantity is often queried for Unit just to see if it has a dimension.
Ask Quantity directly using isDimensionless() method and modify that
method not to care about Quantity value validity; no user was ever
asking for value validity.
