Improve function s2dv_cube

Some improvements:

• Improve function s2dv_cube() that accepts dates of class 'Date':

  • In documentation description of Dates only objects of 'POSIXct' class are accepted. We need to develop the function in order that it can accept also class 'Date'.

• Suppress unnecessary warnings. For example, if metadata is not provided, don't put a warning.

• Add (original?) data frequency as metadata