Performs a normalization in a volume according to a  Region Of Interest
  (ROI)  specified by a binary mask.
  By default,
  mean=0 and std=1 are enforced inside the region defined by the mask
       particle:            - 3d map
                            - filename containing an EM map
                            - cell array ({}) of files or maps
       mask:                - 3d binary map
                            - 1d array with the indices of notzero elements
                            - filename containing a density map
                            - default: full definition domain of particle
       norm:                normalization parameters as couple [mean,std]
                            - default: [0,1]
      particle_normalized:   normalized particle as 
      num_nan:               how many NaN were found inside the mask
      mean:                  original mean on the ROI.
      std:                   original std on the ROI.
      A,K:                   normalization constants that one would use
                             to operate explicitely the conversion:
                             particle_norm = A*particle - K;
       particle_normalized = dynamo_normalize_roi(particle,mask);
     * NaN Values
       NaN values will be ignored.
     * The normalization constants are simply a shorthand for the 
       simple relationships:
          A = 1/std;
          K = mean/std;
       or inversely) 
          std  =  1/A;
          mean =  K/A;
        Thus, their physical meaning is just:
        particle_normalized = A*particle - K;