Development of new spread to error ratio function

Hi @vagudets and @abatalla

This development is ready to be merged.

It includes a new function to calculate the spread to error ratio along with its statistical significance.

This is related to this merge request in SUNSET: https://earth.bsc.es/gitlab/es/sunset/-/merge_requests/141

Thanks,

Nadia

FYI @nperez @cdelgado