Add time_dim and member_dim in Calibration function

Hi @nperez,

The current version of the Calibration() function only accepts 'sdate' as the time dimension and 'member' as the member dimension. It would be good to make it more flexible and let the user choose the name of these dimensions (as, for example, in the s2dv functions) with 'time_dim' and 'member_dim' parameters.

In addition, I have seen that it is mandatory that the 'obs' parameter has 'member' dimension with length=1. I would suggest removing this restriction (of course, in case it makes sense. Maybe I'm missing something), and let the 'obs' have the same dimensions than 'exp' except the 'member_dim' dimension.

Thank you very much in advance,

Carlos