p-winds v2.0.1 beta

This is the beta version of p-winds version 2.0.1. The main changes in this version are:

• The addition of new functions in the hydrogen and lines modules that include the capability to model transmission spectra in the Balmer-series lines of hydrogen. This implementation is based on the formulation laid out in Wyttenbach et al. (2020). Thanks to @jseideleso for contributing to this!
• Other small improvements and bug corrections.