Improve CST_SaveExp warnings, filenames and Dates dimension acceptance

Due to the usage within the Climate Services Verification Suite there was the need of some improvements:

• When the filename is taken from Dates, the format has been corrected to: <var_name>_YYYYmmdd.nc. Before, if the Dates had time values included, they were included in the filename, which was not correct.

• If Dates array has more dimensions additionally to sdate_dim and ftime_dim but they are of length 1, they are dropped.

• Some Warnings are also improved:

  • "Parameter 'startdates' should be a character string containing the start dates in the format 'yyyy-mm-dd', 'yyyymmdd', 'yyyymm', 'POSIXct' or 'Dates' class." to "Parameter 'startdates' should be a character string containing the start dates in the format 'yyyy-mm-dd', 'yyyymmdd', 'yyyymm', 'POSIXct' or 'Dates' class. Files will be named with Dates instead.". and
  • "Spatial coordinate names do not match any of the names accepted by the package." to "Spatial coordinates not found.".
  • The warning: Detected unknown dimension: region, sday, sweek has been removed.

@nperez Could you run your code with this development? You can source the function here: https://earth.bsc.es/gitlab/external/cstools/-/raw/develop-CST_SaveExp_sdates.nc/R/CST_SaveExp.R

Thanks!