Improve print method for 's2dv_cube' objects

Hi @aho, @nperez, @vagudets, @allabres and others,

I open this issue because I think it would be nice to have a different print method for the object class 's2dv_cube'. Now, if we want to see the object it's difficult to understand the structure when we print it in the terminal.

I propose to modify the 'print' method in order to see something similar to this:

> lonlat_prec
's2dv_cube'
Data          [ 1.2009e-08, 5.6514e-09, 0, 8.4771e-09, 2.6138e-08, 2.4018e-08, 0, 6.3578e-09, 1.7449e-07, 1.4906e-07 ... ] 
Dimensions    ( dataset = 1, member = 6, sdate = 3, ftime = 31, lat = 4, lon = 4 ) 
Coordinates  
   dataset : exp1 
   member : 1, 2, 3, 4, 5, 6 
   sdate : 20101101, 20111101, 20121101 
   ftime : 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 
   lat : 47, 46, 45, 44 
   lon : 6, 7, 8, 9 
Attributes   
   varName  : prlr 
   metadata :  
      prlr 
        units : m s-1 
        long name : Total precipitation 
      lon 
      lat 
   Datasets  : exp1 
   Dates  : 1298937600 ... 
   when  : 1681225558 
   source_files  : /esarchive/exp/ecmwf/system5c3s/daily_mean/prlr_s0-24h/prlr_20101101.nc ... 
   not_found_files  :  
   load_parameters  : prlr ... 

What do you think?

Regards,
Eva