Allow different independent variables (apart from time) for drive cycles

In Altoona bus testing, the prescribed speed consists of open-throttle accelerations to a cruise speed with stops designated at certain distances.

CBD/Arterial/Commuter (fuel economy testing):

• Based on on Transit Coach Operating Duty Cycle (ADB Cycle) Fuel Economy Measurement Test (Engineering Type) For Trucks and Buses: SAE 1376 July 82
• “Mile” = 5042 ft / 1 lap – adjusted cruise distance and speed to compensate
• CBD:
  • 2 miles, 7 stops/mile, cruise 20mph
• Arterial:
  • 2 miles, 2 stops/mile, cruise 40mph
• Commuter:
  • 4 miles, 1 stop (at end), cruise 40mph
• “full throttle acceleration to cruise speed”

This is difficult to model with FASTSim's current setup where time is the independent varaible, so we should implement some kind of way to have the distance (i.e. achieved distance) be the independent variable. Would make sense to have this be extensible to the rest of the drive cycle parameters.