BUG: Correct scan_type for NOAA HH Sigmet files.

The scan_type for radar files from the radars on the NOAA Hurrican hunter
P-3 aircraft are correctly set using the antenna_scan_mode parameter in the
Sigmet file header.
Merge pull request #200 from jjhelmus/bug_p3_scan_type

BUG: Correct scan_type for NOAA HH Sigmet files.