Change usage

[ljlamarche]

Code currently is run by following the instructions in HOWTOUSE.txt, which directs users to modify and config file and then provide both that config file and a partial file path as command line arguments to derive_from_fitted.py.

python derive_from_fitted.py isinglass_test.ini 20170215.005/20170215.005_lp_1min-fitcal.h5

This is awkward. Instead, make this a pip installable package that can be run with a single config file from the command line.

resolvedvelocities2d config.ini