Hn2016-falwa

Enhancement in functionality
- The computation of LWA, reference state and vertically averaged fluxes in the **Southern Hemisphere** is now available in the `QGField` class!
- The computed variables can be accessed via the QGField class without calling any methods, e.g. the 3D wave activity can be retrieved from the QGField object `qgfield` by `qgfield.lwa`. See documentation/example for all the attributes.
- Added more sanity checks for the inputs of `QGField`.

Other changes
- The documentation page is fixed and now available at [http://csyhuang.github.io/hn2016_falwa](http://csyhuang.github.io/hn2016_falwa).
- Added test data and increase unit test coverage for `QGField`.

- Fixed a bug in the interpolation procedure when the input latitude grid is of even number of points
- Updated the notes regarding the equation solver for the reference state (in `notes/`)

- Replaced `unittest` with `pytest` for more flexibility in testing
- Improved the encapsulation of variables in the module `oopinterface`, e.g. methods like `qgfield.get_lwa()` are replaced by `qgfield.lwa`.

- Enhanced functionality of the class QGField to process latitude grids with even number of grid points.
- Enhanced interface of QGField with some functions to retrieve computed quantities at intermediate steps. See documentation of QGField for details.

- Fixed an issue in compute_reference_states.f90 that can potentially lead to segmentation fault error in computing `tbar` (issue 13).
- Added unit tests for `qgfield.compute_reference_states`.

- Added a function `zonal_convergence` in `hn2016_falwa/utilities.py` that it computes the zonal component
of convergence in spherical coordinates
- Added unittest for `utilities.zonal_convergence` in `tests/test_utilities.py`
- Updated the demo scripts in `examples/nh2018_science/` that it now computes LWA, LWA flux and their convergence/divergence
and saves them in a netCDF file for 8 days (6-hourly data) in 2005.
- Included a conda environment specification file `examples/nh2018_science/environment.yml` that enables one to
run all the scripts with visualizations in the repository
- Included `examples/README.md` with brief descriptions of the sample scripts
- Updated descriptions in `examples/nh2018_science/README.md`