ENH: pulse_width included when reading Sigmet files.

The radar pulse width is extracted when reading Sigmet files and
included in the instrument_parameters attribute of the returned
Radar object under the 'pulse_width' key.  The 'data' array contains the pulse
width in seconds for each time point, these widths are always the same.
Merge pull request #170 from jjhelmus/enh_sigmet_pulse_width

ENH: pulse_width included when reading Sigmet files.