Add warning for default climatology when ref is null in RMSSS.R and MSSS.R

This merge request is to add a warning to the RMSSS.R and MSSS.R functions addressing the use of a default climatology of 0 when ref is NULL (!182 (merged)), before the next s2dv release. Assuming a climatology of 0 is appropriate when the input data (exp and obs) are anomalies, but leads to incorrect RMSSS values when the input data is full-field.

FYI @vagudets