Update acf_gearbox.lua

Addition of Double Differential transmission.
Create DoubleDiff

Addition of the Double Differential Transmission.
Rename DoubleDiff to doublediff
Update acffunctions.lua

Addition of "SteerRate" E2 function for Double Differentials.
Rename doublediff to doublediff.lua
Update doublediff.lua
Update doublediff.lua
Update acf_gearbox.lua
Update doublediff.lua
Update doublediff.lua
Update acf_gearbox.lua
Update acf_gearbox.lua
Update acf_gearbox.lua
Update acf_gearbox.lua

self.GearRatio is a bit more appropriate than dividing by 20, but I still think that isn't a perfect solution.
Update acf_gearbox.lua

Not sure what happened and why this works now, but I was testing today and it worked perfectly fine, this is very RPM dependent and gear ratios need to be tuned very precisely. Also, it seems that adjusting the gear/final drive on the double diff itself isnt the best option.
Update acf_gearbox.lua
Update acf_gearbox.lua
Merge branch 'master' of https://github.com/Scottyprogrammer/ACF