BUG: MdvFile looks at first field header
TST: small example mdv and netcdf files

These example files will be used to test the Py-ART io modules.
The scripts used to create and visualize these example files are
included in the testing/data directory.
ENH: reading of Sigmet/IRIS files
MAINT: sigmet refactor and docs
Merge pull request #68 from jjhelmus/master

ENH: Reading of Sigmet/IRIS files