Fix bug when converting unit with base factor other then 1. (ie. Kilogram, Metric Ton.)

While computing the scaling factor (the ratio of both quantity value),  the factor type defined in the unit definition was used, which is incorrect in that case where base unit is not aligned on the simplest symbol (like gram vs kilogram).

At that point, the library is quite stable and is performing as it was originally planned more than ten years ago.

More to go... Liter next. Hopefully, this will go through quite seamlessly.
Restored the SquareMeter and CubicMeter unit test.

Fix operator* and operator/ that were not working properly with exponent.
Added Liter volume unit.

After the last fixes, including the management of exponent in operators, this step went through seamlessly as previously hoped. Hurray!
Added a batch of SI derived unit.

Also, placed SI derived unit accepted by the SI in the SI namespace for convenience. That leaves the NonSI namespace for unit out of the scope of the SI Standard such as Inch, foot, and the dreaded Fahrenheit unit that I will add next.

Source used for the added SI derived unit : http://www.checklist.org.br/d/internationalsystemofunits.pdf