ENH: CHL file reader incorporated into Py-ART.

This reworks the `read_chl` function to better fit with Py-ART's other read functions. The default field name mappings and metadata are moved to the **default_config.py** file.

Additionally:
* The `pyart.io.read` function can detect and read CHL files.
* chl.py and the `pyart.io.read_chl` function are included in the documentation.
* Unit tests for chl.py with 100% code coverage using a small CHL file created by selected two rays from a larger full size CHL file.