Update and make visible .make.eval.train.dexes

Hi @vagudets, @abatalla and @nperez

Hi,
I opened this issue to update the make.eval.train.dexes() function, that is part of CST_Calibration.R in https://earth.bsc.es/gitlab/external/cstools/-/tree/dev-cross-indices?ref_type=heads. The updates follow from issue #150. Following are the updates to implement:

• make this function visible to users.
• use a simpler function name.
• unify methods leave-one-out and k-fold to leave-k-out, check if k is odd
• don't preserve 'NA' values in the result of method retrospective.

Thanks
Theertha K