p-winds

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.If you use this software, please cite it as below