cgs unit system

Hi, thank you for your awesome math library!

I am thinking about taking advantage of this library and build a calculator for physicists and astronomers. Since cgs unit system is still widely used in the astronomy community, I'd like to help to add a cgs unit system to this library.

I found `UNIT_SYSTEMS.cgs` but didn't find a `.toCGS()` function. Here's what I plan to do:
1. Complete `UNIT_SYSTEMS.cgs`: add a missing basic unit 'Ba = g/(cm⋅s2)'; implement the ESU units system for Electromagnetics.
2. Define `Unit.prototype.toCGS`: copy `Unit.prototype.toSI` and change every instance of `UNIT_SYSTEMS.si` to `UNIT_SYSTEMS.cgs`.

Please let me know what do you think, or what is already in math.js that I missed.