Improve as.s2dv_cube

There are differences in element $Datasets when the object come from Load or from Start:

• From Load():

  ..$ Datasets       :List of 1
  .. ..$ exp1:List of 2
  .. .. ..$ InitializationDates: POSIXct[1:18], format: "2010-11-01" ...
  .. .. .. ..- attr(*, "names")="Member_1" ...
  .. .. ..$ Members            : chr [1:6] "Member_1" "Member_2" "Member_3" "Member_4" ...
  .. .. ..- attr(*, "dataset")= chr "exp1"
  .. .. ..- attr(*, "source")= chr "/esarchive/exp/ecmwf/system5c3s/daily_mean/prlr_s0-24h"
  .. .. ..- attr(*, "URL")= chr "none"

• From Start():

  ..$ Datasets       : chr "dat1"

Changes

• Only a vector of character strings will be kept, following Start() development.
• Element 'dataset' will be added in the corresponding coordinate inside $coords