Radiometric drift for each band

Change radiometric drift routine so that it now
loops over all the bands for the target sensor,
and looks to see if there is a matching band for
the reference sensor (bands are considered matching
if within 10mn). Then calculate the drift for each
band where a match was found.
Save drifts to text file

Write a csv file with the drift for each band.
Also add a unit test for this method.
Changes to rad drift unit test

Make test_plot_drift check the drift return
value. Also, in the main method set a logical
parameter we can use to turn off plotting
commands when running the tests (since the
backend isn't defined on the build server)