Included cross-validation in score climatologies

Hi @vagudets,

This is a small development to include the cross.val parameter in the calculation of the metrics: RPS_clim and CRPS_clim.

I have modified the following three files for this development:

• modules/Skill/R/CRPS_clim.R
• modules/Skill/R/RPS_clim.R
• modules/Skill/Skill.R

This is ready to be merged. Let me know if you have any questions.

Thanks,

Nadia