Hn2016-falwa

- 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.

List of changes (Feb 26, 2018):
- hn2016_falwa/beta_version.py: In *solve_uref_both_bc*, a plotting option is added.
- hn2016_falwa/wrapper.py: In all functions, when n_points are not specified, it is taken to be nlat_s (input). Also fixed a bug of missing argument n_points in *theta_lwa*.
- hn2016_falwa/utilities.py: In *static_stability*, make s_et and n_et an integer if they are not input. In *compute_qgpv_givenvort*, remove the bug that nlat_s is being hard-coded by mistake.

The package has been significantly restructured. The file api.py has been removed. Functions are classified into 4 different categories according to their structures.

For existing users: instead of using
python
from hn2016.api import function

please switch to, for example, if you want to use the wrapper function *qgpv_eqlat_lwa*
python
from hn2016 import wrapper
qref, lwa = wrapper. qgpv_eqlat_lwa(...)


Existing functions are categorized into 4 types: basis functions (*basis*), wrapper functions (*wrapper*), utility functions (*utilities*) and beta-version functions (*beta_version*). Please refer to the [documentation](http://hn2016-falwa.readthedocs.io) for the lists.

This version 0.1.7 has some major updates in function names and library structure.

Major updates:
- Name of functions are all now in **small letters**.
- The radius of planet (*planet_radius*) is now an optional input for the functions (default value: Earth's radius).
- The function *static_stability* can now take in 2D (i.e. zonal mean) or 3D field of potential temperature.
- The syntax in the sample IPython notebooks are updated. (see *example/*)
- Unittest directory has been set up (*tests/*) and will be constantly updated.
- A new function *theta_lwa* has been added to compute surface wave activity (See Huang and Nakamura (2016). It is the **longitudinally local** version of B^* in Nakamura and Solomon (2010) / Wang and Nakamura (2015)).

Note for existing users
- Please change all characters in the names of functions into **small letters**.
- The directory of functions can be access via *hn2016_falwa.api* instead of directly from *hn2016_falwa*.
- The old function *Solve_Uref_noslip* has been renamed *solve_uref_both_bc*. The input fields are the same. The dimension-dependent parameters are functions of input instead of hard-coded in the previous version. Thanks slubis for reminding.

Added two new functions that allows computation of LWA based on *prescribed* equivalent-latitudes:
- barotropic_input_Qref_to_compute_LWA
- qgpv_input_Qref_to_compute_LWA