Adding RPC-based calibration method

See merge request !94

Sorted the factor levels in the order required in the legend to avoid problems later. Reorganized ggplot calls.

added na.rm, apply_to and alpha parameters and their checks. Changed na.rm=T for na.rm=na.rm everywhere. Added documentation.

Vignette and CDS_Seasonal_Downloader

See merge request external/cstools!93

New parameters and change to checks: 'memb_dim' and 'sdate_dim'

See merge request !92