Add 'sign' parameter to Corr()

Hi @aho

I ended up adding 'sign' to Corr() as detailed here https://earth.bsc.es/gitlab/es/s2dv/-/issues/79#note_182291. I also changed 'conf.lev' to 'alpha' which is what's causing the pipeline to fail right now.

As soon as I can I plan on testing this development with Auto-S2S.

Let me know if you need anything else from me.

Victòria