Moved code that should be common to all throttles back to parent Throttle class

Previously the pot throttle had code to map throttle position to output throttle but this mapping is common to all throttle types. So, it is now separated out and placed in the parent class. Also, the setter functions were moved. Additionally, Throttle now has virtual functions for getting the raw throttle values. Child classes can override these.
Updated licensing terms in the readme and all source/header files. Hurray for lawyers.
Moved throttle detection/calibration into Throttle