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.