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.