ENH: Initial support for reading ODIM_H5 files.

* Support for reading ODIM_H5 file containing PPI scans using
  `pyart.aux_io.read_odim_h5`.
Merge pull request #195 from jjhelmus/odim_h5

ENH: Initial support for reading ODIM_H5 files.
BUG: relative import in odim_h5 module.

Corrected bug with invalid import of Radar object by replacing
absolute imports with relative imports.
Merge pull request #196 from jjhelmus/odim_h5

BUG: relative import in odim_h5 module.
ENH: Reading in gzip and bzip2 files.

The contents of gzip and bzip2 files can be detected and read using
the pyart.io.read function.
Merge pull request #199 from jjhelmus/read_compressed

ENH: Reading in gzip and bzip2 files.