Add comment to explain why CPS appears to output velocity instead of displacement
Refer to line: https://code.usgs.gov/ghsc/lhp/lsforce/-/blob/main/lsforce/lsforce.py#L227 This is because CPS Green's functions are ground velocity for a step function to help with stability at low frequencies. -V flag just keeps Green's functions in their original form, and the -OD flag tells it to call the output displacement. This is because ground velocity for a step function is the same as ground displacement for a delta-like function.