gets the basic info on a data folder
 
  INPUT
 
   source:         a char expression stating from where the particles will
                   be read. Possibilities are:
                   1) the name of a folder containing Dynamo-style particles
                   2) a regular expression
                   3) a virtual project
                   4) the name of a project
 
 
   Parameter/value
 
  - - - - - - - - - - - - - - - - - - -  - - - - - - - - - - - - - - - -
 
   Format of individual files 
   --------------------------
 
   'source_type','st'  : 'folder' , 'regexp' or 'guess'
                          - default: 'folder'
 
   'volume_type','vt'  : 'particle' or 'pfmask'
                          - default: 'particle'
 
   'pad'               : looks for particle names with a concrete 
                          zero-paadding scheme:
                          - default: 5 (standard Dynamo convention)
 
  - - - - - - - - - - - - - - - - - - -  - - - - - - - - - - - - - - - -
 
   Output
   ------
 
     'all'              : logical.
                          in interactive sessions: orders a more exhaustive
                          level of information detail.
                          This includes
                          - normal info:
                          - info on pfmask files
                          - checks if the folder is a compaction on any
                            other folder.
   
     'getfiles'         : logical
                          includes a field containing a cellarray
                          with the names of the retrieved files.
 
 
     'sel'              : writes the retrieved files into a sel
 
    
     'ws'               : stores the output into a command line variable
                          
 
  - - - - - - - - - - - - - - - - - - -  - - - - - - - - - - - - - - - -
 
   'v'                 : verbosity
                     
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
   OUTPUT to command line
 
     facts:
 
    is a structure with the following fields:
 
   .folder_name
   .correct 0 or 1
   .N                number of particles.
   .tags             tags of valid particle file.
   .Mb               size in Mb of one particle.
   .Mb_all           size in Mb of the whole folder.
   .l                sidelength of the cube,
   .source_type      how the files will be are formatted  'dynamo_folder'/'regexp_files'
   .extension        file extension of the particles.
                     
   .N_pfmask         number of files of type 'pfmask' that are also
                     in the scanned fodler
                     (this field appears only when scanning for 'particle'
                      type files)
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
 
   EXAMPLE
 
     % create a simple data  set
       dtutorial testdata -M 8 -jd on;
 
       dynamo_data_info('testdata');    
 
       ddinfo testdata;
 
     % Exhaustive information:
 
       ddinfo testdata -all on;