BUG: read_sigmet has wrong fixed_angle data type

Using the read_sigmet routine followed by write_cfradial now results in
the fixed_angle variable having a float type as apposed to a double.
Merge pull request #149 from jjhelmus/master

BUG: read_sigmet has wrong fixed_angle data type
ENH: Automatic inclusion of time_reference

netCDF files written with pyart.io.write_cfradial now include a time_reference
variable.  The inclusion of time_reference in files can still be controlled
using the **time_reference** keyword parameter.
Merge pull request #150 from jjhelmus/master

ENH: Automatic inclusion of time_reference
ENH: beam width parameters when reading RSL files.
ENH: beam width parameters when reading Sigmet files
ENH: beam width parameters when reading MDV files.
BUG: writing cfradial file without instrument_parameters

Writing a Cf/Radial file from a radar object in which the instrument_parameters
attribute is None no longer errors out.
MAINT: Error message when reading nexrad msg1 files
Merge pull request #151 from jjhelmus/master

ENH: beam width parameter included when reading files
Merge remote branch 'upstream/master'