Falwa

- 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`

- Unit tests for all functions in `basis.py` and the interpolation method of `QGField` in `oopinterface.py` are included.
- Fixed a typo in `hn2016_falwa/examples/nh2018_science/demo_script_for_nh2018.ipynb`. Now, the timestamps in the figures are consistent with the source of data. Also, the latex symbols in the figure titles are fixed.
- Modified a procedure in `eqvlat` in `basis.py` such that division by a zero differential area is avoided.