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  FUNCTION save2Dbin( fpath, fname, nx, nz, data);
  This function simply saves a 2D matrix to a binary file. It requires the
  following info: - fpath : location of the binary file
                  - fname : name of the binary file
                  - nx    : amount of elements in x-direction
                  - nz    : amount of elements in z-direction (or y-)
                  - data  : 2-D data stored in the file.
  
  By Jacques Ernst (2006)                                        ETH Zurich
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