updating cal_bev.py to bring it up to speed with cal_hev.py

[robinsteuteville]

I updated everything I could in cal_bev.py. A couple notes of places where I was unsure:

1. I didn't populate cyc_files_for_cal since I wasn't sure which files you would want to have for the calibration vs. validation. Note that unlike the Hyundai HEV files, the Bolt files seem to combine multiple different scenarios within the same text file -- not sure if this changes how we need to handle things.
2. In the veh_init() function, I replaced the Hyundai data header HVBatt_SOC_high_precision_PCAN__per with a similar column name from the Bolt data, HVBatt_SOC_CAN4__per. I'm not positive this is the correct column, but from what I could tell it was the only column in the Bolt .txt files that included SOC in the column name.