Extract doublets

Add function to libtools to extract doublet pairs
(pairs that match AMC, time, and ROI coverage
criteria).

The function loops through all images from the
target sensor, and finds any reference images
that make a valid doublet with that target image.

Returns lists of the indices of the image pairs in
the original arrays, and a list of the mean time
of each pair.
Calculate and plot radiometric drift

Using the new doublet extraction, get list of
doublets, then extract the corresponding
timeseries of reflectances, calculate the ratio
of target/reference reflectance, and plot it.
Fit a straight line and plot that too.

(nb at the moment, the bands are hardcoded)
Add unittest for brdf filter_timeseries method