ladsantos/p-winds: p-winds v1.1

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