Add USC volume two reader

[emthompson-usgs]

We already have a USC V2 file in the tests directory here: https://github.com/usgs/groundmotion-processing/blob/master/tests/data/usc/ci3144585/017m30bt.s0a But the tests conspicuously ignore it, which is probably because the USC reader only supports V1 processing.

[emthompson-usgs]

Additionally, we had bugs in how velocity and displacement were getting treated in the dmg v2 format. So I disabled those but I think it makes sense to address this issue at the same time. For future reference, the file that exposed the incorrect handling of velocity and acceleration is 24592_LosAngeles_CityTerrace_LA_CITY.V2 for the Northridge earthquake. Would be good to use this for testing when we address this issue.