BUG: scan_type properly determined when reading file with RSL.

* The scan_type is detemined by examining the RSL Radar header scan_mode
  element.  This resolves some issue where UF files are read as the incorrect
  type as the sweep elev and azimuth are no set correctly.
* The fixed angles are set from the fix_angle element of the first RSL Ray
  in each sweep using the new get_sweep_fix_angles method of the Volume class.