readSnP

      readSnP   Reads a touchstone file, returning a frequency vector and
                S-parameter matrix. If freq_desired is provided, S-parameter
                data is interpolated and then returned. Units of freq_desired
                must be in Hz.
          [S,freq] = readSnP(path,freq_desired,kind)
          input:
              path: [string] path/to/filename of touchstone file (*.SnP)
              freq_desired (optional): [num_pts] frequency vector (in Hz)
                  for interpolation
              kind (optional): [str] Specifies the kind of interpolation
                  ('MA', 'DB', or 'RI') Function interpolated to frequency desired
                  using linear interpolation on the magnitude and phase ('MP'),
                  magnitude in dB and phase ('DB'), or real and imaginary parts ('RI').
                  default is 'DB'
          output:
              S: [1,num_pts] or [n,n,num_pts] for 1-Port or n-Port S-parameter
                 matrices. ex. S21 over frequency can be accessed as S(:,2,1)
              f: (optional) [num_pts] frequency vector (in Hz) of the data.