Calculate error estimates

Add calculation of error estimates to the timeseries
method. Then just return the result arrays, and split
out the plotting to a separate method.
Return mean dates from the binned timeseries

Instead of storing the mid point of the bin,
calculate the mean date of the included images.
Do this by converting to unix timestamp, so
they are just integers which we can average.

This gave an error when the bin size was such
that we had some bins with no images found,
leading to an empty timestamps list. So add a
check that we found some images before doing
any of the brdf calculations.
Improve the timeseries plot

Tidy up the plot a bit:
- Make the figure window bigger
- Add (optional) title, xlabel and ylabel
  arguments, and use instrument and region
  in our title.
- Move the legend box outside the plot area,
  and add a title to it
Add some tests for the BRDF code