Merge pull request #168 from jjhelmus/master

BUG: Fixed two bugs in chl files.
MAINT: Created aux_io namespace, moved read_gamic.
MAINT: Radar and Grid objects moved to core namespace.

Closes #165
TST: Doc building is hushed when not in ARM-DOE repo.
TST: removed aux_io from coverage report.

This will increase the total coverage of the code since modules in
pyart/aux_io are excluded from the statement count.  Since these modules
explicitly state they may not be tested this seems reasonable.
Merge pull request #169 from jjhelmus/aux_io

MAINT: pyart.aux_io and pyart.core name spaces created.
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.