Develop clim scores

Hi @vagudets

To test the module Indices that I am developing and to ensure it works for the scorecards, we need to calculate additional metrics. I have created this merge request for that. Nadia has already tested that she can get the new metrics in a saving file. Additionally, we have added a parameter cross.val because we will need it as well.

Skill:
  metrics: rps crps rms
  cross.val: yes

The cross.val parameter may be ingested for more functions. I have included it in the ones I know.

It would be nice to have this development in the master.

Thanks,

Núria and @nmilders