Added ability to set ratio of a CVT

Users can now directly set the gear ratio of a CVT.  For example, this
allows use of custom algorithms for CVT ratio, E2 controlled CVTs that
act as 6 speed gearboxes with multiple settings (sport, race, etc), and
dual-CVT tank turning via different ratios.  Set the ratio to 0 or
negative for the CVT to resume using target min/max rpm calculations for
ratio.
Merge pull request #30 from Fervidusletum/CVT_RatioInput

Added ability to set ratio of a CVT