RPS and RPSS

Hi @aho,

I have included two new functions in the branch develop-RPSS of s2dv:

• RPS(): computes the Ranked Probability Score, which is useful to estimate the quality of a probabilistic forecast. The function is in the following link: https://earth.bsc.es/gitlab/es/s2dv/-/blob/develop-RPSS/R/RPS.R

• RPSS(): computes the Ranked Probability Skill Score, which is based on the RPS and is useful to compare the quality of two probabilistic forecasts. The statistical significance is estimated with s2dv::RandomWalkTest(). The function is in the following link: https://earth.bsc.es/gitlab/es/s2dv/-/blob/develop-RPSS/R/RPSS.R

For the s2dv::RandomWalkTest(), I realised that there was a typo in the documentation so I corrected it.

The functions already have documentation, basic checks, and they are in an s2dv-like format. Could you please have a look at them and let me know if I have to add/modify anything?

Thanks a lot!

Carlos