writeSnP

      writeSnP    writes S-parameter matrix to touchstone (*.SnP) file
          [] = writeSnP(freq_vec,S_data,path,freq_unit,kind)
          input:
              freq_vec: [num_pts] frequency vector of measurements
                                  in units of freq_unit.
              S_data: [1,num_pts] or [n,n,num_pts] for 1-port or n-port
                       S-parameters respectively
              path: [string] path/to/filename
              freq_unit(optional): [string] units of the frequency vector
                         options ['HZ','KHZ','MHZ','GHZ'] (default: Hz)
              kind (optional): [str] Specifies the complex data format in
                         file. Options are Magnitude and phase ('MP'), magnitude in
                         dB and phase ('DB'), or real and imaginary parts ('RI').
                         default is 'DB'