option to apply uniform scaling to velocity

[carltape]

Currently we have options to remove instrument response or keep the raw files. It may be useful to have something in between: apply a scale factor (relevant for some fixed freq) that converts from counts to velocity (or acceleration). This would be similar to the 'calib' factor that is applied by default in the GISMO Waveform Toolbox for Matlab. Not sure if we want to implement this right now. Below are some lines from Vipul that seem good.

r=stations.get_response('AK.PPLA..BHZ',otime)
r
Channel Response
From M/S (velocity in meters per second) to COUNTS (digital counts)
Overall Sensitivity: 4.8876e+08 defined at 0.200 Hz
3 stages:
Stage 1: PolesZerosResponseStage from M/S to V, gain: 1165.3
Stage 2: CoefficientsTypeResponseStage from V to COUNTS, gain: 419430
Stage 3: CoefficientsTypeResponseStage from COUNTS to COUNTS, gain: 1
r.response_stages[1].stage_gain
419430