P-winds

This is a patch to the stable version of `p-winds`. Here are the changes:

- Corrected a bug in `hydrogen.py` where the code passed velocity and radii to `parker.average_molecular_weight()` in units of values at the sonic radius instead of km/s and Jupiter radii. This correction has only a small impact to the resulting models.
- Improved rendering of the documentation in ReadTheDocs.
- Some small updates in the test suites.

This patch corrects several small bugs in the code (see issue 36).

This patch contains several optimizations for `p-winds` and some new features. We highly recommend users to update. Changelog:

* Small optimization in `parker.py` when running `structure()` and `structure_tidal()`
* Added options to return reaction rates in `hydrogen.py` and `helium.py` when calculating the H ion fraction or He population fraction. See an example in the documentation (more specifically the Quickstart notebook).
* Added a function to return the He ion fraction in `helium.py` (does not calculate the singlet/triplet He atoms)
* Removed an annoying warning that could appear when using MUSCLES spectra generated from `tools.py`
* Some functions that were previously implemented in `microphysics.py` were moved to `lines.py`. See the documentation to learn how this affects your own codes.
* Significant optimization on `helium.population_fraction()` that cuts down computation of He profiles by a factor of ~50% and improves numerical stability of the solution.
* Added an option to manually set the grid resolution of the line-of-sight direction when using `transit.radiative_transfer_2d()` with the parameter `z_grid_size`. Grids with lower resolution than the standard on in the code will accelerate the computation of the `'formal'` absorption profiles. It does not affect the `'average'` method.

This is a patch that corrects a critical bug in `p-winds` 1.3. In previous versions, there was an error in the calculation of `density_r` in `structure_tidal()` that yielded strange, but still reasonable results. The calculation in question is that of Eq. D10 in [Vissapragada et al. (2022)](https://iopscience.iop.org/article/10.3847/1538-3881/ac92f2). The mistake was in the implementation of the second term of this equation. It has been corrected in this patch, version 1.3.1.

Please update your code to match this correction.

This is the stable version 1.3.0 of `p-winds`. Here is the changelog:

- `parker.py` was updated with two new functions: `radius_sonic_point_tidal()` and `structure_tidal()`, which calculate the structure of the upper atmosphere while taking into account a correction due to tidal effects.
- `hydrogen.py` was updated to allow the usage of the tidal correction implemented in `parker.py`.

As always, please check the documentation of the code for guidance on the functions parameters.

This is a beta test of the version 1.3.0 of `p-winds`. Here is the changelog:

- `parker.py` was updated with two new functions: `radius_sonic_point_tidal()` and `structure_tidal()`, which calculate the structure of the upper atmosphere while taking into account a correction due to tidal effects.
- `hydrogen.py` was updated to allow the usage of the tidal correction implemented in `parker.py`.

We encourage users to test this beta version and report bugs in the [GitHub issues](https://github.com/ladsantos/p-winds/issues). As always, please check the documentation of the code for guidance on the functions parameters!