Refactor calculation of sensor drift

Move the method that calculated drift timeseries
from the Radiometric drift module to libtools, so
we can use it in the supersensor module too. Also
move the associated unit test.

At the same time, change so that instead of just
reference/target, it calculates (target-reference)/reference
and simplify to use a list comprehension instead of
a loop with several lines.
Fit polynomial to doublets

- Loop through sensors to be recalibrated
- Loop through the reference sensor bands
- If the current recalibration sensor includes
  this band:
     - Read reflectance data for both sensors
     - Find doublets
     - Calculate timeseries of:
       (calibration TOA - reference TOA)/reference TOA
     - Fit polynomial to this timeseries and
       return the coefficients
Add recalibration

First refactor code to change reference to the
"calibration sensor(s)" to "target sensor(s)"
to avoid confusion.

Add method to recalibrate the target sensor
at all dates, using the polynomical that was
calculated from the doublets. This is
effectively saying "given this target sensor
measurement, what measurement would we have
got from the reference sensor?"

Also add a unit test for the new method