P-winds

Version 1.1.1 contains an important patch for [v1.1.0](https://github.com/ladsantos/p-winds/releases/tag/v1.1.0).

New in this version:
* Corrected a bug in the calculation of constants `k1` and `k2` in `hydrogen.ion_fraction()`.

We have recently received the referee report of our manuscript submitted for peer-review on Astronomy & Astrophysics. The paper has been recommended for publication after a revision, which we are currently working on. `p-winds` version 1.1 includes several important updates following up on the referee report and on another paper our team is currently working on.

New in this version:
* Changed some nomenclature to use H fraction instead of H/He or H/He fraction, which is more accurate. This implicates in some syntax changes when calling functions like `parker.sound_speed()`, `hydrogen.ion_fraction()`, and `helium.population_fraction()`. In case of doubt, always check the documentation!
* Implementation of an exact calculation for the photoionization of H instead of using the approximation of Oklopčić & Hirata (2018). Suffices to pass the parameter `exact_phi=True` in the function `hydrogen.ion_fraction()`. No further extra coding is necessary, everything happens under the hood.
* The calculation of photoionization rates of H and He using monochromatic fluxes now accepts as input the average ionizing photon energy or wavelength. Default values are based on the solar spectrum.
* SciPy requirement updated to v1.5 or higher.

After a long period of development, we are proud to release the first stable, production-level version of `p-winds`! The code was extensively tested in research-grade applications, and a manuscript is going to be submitted for peer-reviewing very soon.

New in this version:
- A few improvements in the documentation

p-winds` is a Python implementation of Parker wind models for planetary atmospheres.

New in the 0.6.4-beta version:

- Added a function in the `tools.py` module to make host star spectra from the MUSCLES Treasury Survey.
- Added a warning about future syntax changes in the `transit.py` module.

p-winds` is a Python implementation of Parker wind models for planetary atmospheres.

New in the 0.6.2-beta version:

- Removed dependency on `pillow` and added [`flatstar`](https://flatstar.readthedocs.io/en/latest/) instead
- `flatstar` handles all the jobs to create arrays for ray tracing
- In previous versions, the calculated optical depths with `transit.py` were wrong, and now they are correct
- Added options to supersample the ray tracing array to avoid hard pixel edges
- Added limb darkening to `transit.py`
- Added planetary wind broadening to the radiative transfer in `transit.py`
- The functions of `transit.py` now take new extra inputs and yield different outputs as well. Check the documentation for details.

p-winds` is a Python implementation of Parker wind models for planetary atmospheres.

New in the 0.5.4-beta version:

- `helium.py` now uses `odeint()` instead of `solve_ivp()` as default to solve the steady-state distribution of helium because it yields results 2.6 times faster. `hydrogen.py` still uses `solve_ivp()` because it is faster than `odeint()` in that case.