BRDF updates

- k coefficients only calculated once, for some given
  date range. Then these values are used to calculate
  BRDF for the entire timeseries.
  (If no date range given, it defaults to using the
   whole timeseries)

- Plot (reflectance - BRDF)/BRDF * 100 (ie the error
  as a percentage of modelled BRDF)

- Before plotting, remove (by setting to nan) values
  that are more than two standard deviations away from
  mean

- Add option to save the plot, if a filename is passed
  to plot_timeseries(). Amend the plot unit test so that
  no file is created there.

- Add measured reflectance to the output csv file. This
  means also re-sampling into the time bins as with BRDF.

- Re-build the html docs to reflect the changes