Ecoflux

Changed

* Updated the project structure to PyScaffold 4.6.
* Applied the [Black](https://github.com/psf/black) formatter.

Added

* An empirical function to calculate the diffuse radiation fraction
`radtrans.canopy_light.diffuse_radiation`.

Changed

* Renamed the project as ``ecoflux``.
* Restructured the project using [PyScaffold](https://pyscaffold.org).

Added

* `setup.py` and `Makefile` to automate the installation.
* `left` and `right` boundary options in `stats.timeseries.simple_gapfill`.

Changed

* Shortened some constant names in `constants.py`.
* Reformatted `light_response.py`.
* Updated `.gitignore`.

Fixed

* An error that occurs when importing `zscore` from `stats.summary`.

Added

* Ecophysiology: Light response functions for photosynthesis.
* Physical chemistry: Saturation water vapor pressure function.
* Radiative transfer
+ Solar angle function
+ Planck's law
+ Canopy light extinction
* Statistics
+ Histogram bin-size calculation
+ Some functions for summary statistics
+ Some time series functions