Gauss system support

Hello

I implemented the support for Gauss units. It is not perfect (it miss the conversions from/to SI and CGS) but I tried to follow the existing design as possible.

Unfortunately, I had to duplicate a set of physical_dimensions (Gauss define electric charge differently): you can find it in physical_dimensions/gs directory

If you want, and you know how to implements the converters, you can integrate to boost/units release. If you think this implementation does not fit the design of Boost Units please just notice me so I can create a standalone project (with only my modification).

The modifications can be found here (branch: gauss)
https://gitlab.com/micrenda/boost-units-gauss/tree/gauss

Regards