P-winds

Changelog:

- Added option to scale MUSCLES spectra to arbitrary stellar radii
- Implemented a `v_guess` routine in `hydrogen.py` to facilitate the convergence of structure solutions

This new version is a patch to v1.2.

New in this version:
* In `helium.py`, corrected a mistake in the equation that calculates the charge exchange process that produces He singlet (`term_19`). This change should have a minimal impact on models that have more extended atmospheres, but they likely have more impact on more compact atmospheres where the charge exchange process is important near the planet's surface. Many thanks to **Andrew Allan** for picking up on this issue!

This new version is an update based on the referee report of the manuscript submitted to Astronomy & Astrophysics.

New in this version (compared to v.1.1):
* `hydrogen.py` requires a first guess for the mean molecular weight as input, but it can be self-consistently calculated by setting `relax_solution` to `True`
* `hydrogen.py` does not require a value for the average H ion fraction anymore
* `parker.sound_speed()` now takes as input only temperature and mean molecular weight
* `parker` has a new function to calculate the average mean molecular weight as in Eq. A.3 in Lampón et al. (2020)
* `relax_solution` now checks for changes in the entire population profile instead of only the outer layer, both in the `hydrogen` and `helium` modules
* Corrected the implementation of turbulence broadening to reflect the same formulation of Lampón et al. (2020)
* Fixed a bug in the exact photoionization calculation
* Corrected a mistake in the calculation of the Guassian broadening term in the Voigt profiles

This is a patch for the pre-release version of `p-winds` 1.2.

New in this version:
* Corrected the implementation of turbulence broadening to reflect the same formulation of Lampón et al. (2020)
* Fixed a bug in the exact photoionization calculation
* Corrected a mistake in the calculation of the Guassian broadening term in the Voigt profiles

Remember to always check the documentation!

This is the pre-release version of `p-winds` 1.2.

New in this version:
* `hydrogen.py` requires a first guess for the mean molecular weight as input, but it can be self-consistently calculated by setting `relax_solution` to `True`
* `hydrogen.py` does not require a value for the average H ion fraction anymore
* `parker.sound_speed()` now takes as input only temperature and mean molecular weight
* `parker` has a new function to calculate the average mean molecular weight as in Eq. A.3 in Lampón et al. (2020)
* `relax_solution` now checks for changes in the entire population profile instead of only the outer layer, both in the `hydrogen` and `helium` modules

Remember to always check the documentation!

Version 1.1.3 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 mistake in the calculation of the Gaussian term in the Voigt profile
* Fixed a bug in the exact photoionization calculation
* Added future warnings for changes in v1.2