exporter usage

usage : ./dump_matrix [options] mat
usage : ./dump_matrix [options] -

 [default] prints the header of the matrix if no option is given
 options can be one of :
     -h : print this help
     -l : dump the whole matrix in SMS format
     -m : dump the whole matrix in magma format
     -s : dump the whole matrix in strict SMS format
 options can be additionally:
     -n : uses new format [default : old format]
 filename argument:
    mat : represents the matrix is the correct format.
    -   : matrix read from stdin as in zcat mat.gz | ./dump_matrix -
warning: filename has to be in the correct format