Allow flexible dimension order in CST_MultiMetric and improve the function

I open this issue to improve CST_MultiMetric function. This function is used in MostLikelyTercile vignette.

Improvements detected:

• Flexible order in exp and obs dimensions
• Allow ensemble member dimension to be NULL in obs.
• Improve initial checks:
  • Order of the checks
  • Some checks are missing
  • Some of them may be repeated in the internal s2dv functions and could be removed.
  • Add dat_dim parameter