Added function to estimate critical speed - critical_speed() (thanks to @mrpetebonner).
Adjusted critical power functions to make sure it works even when column names have spaces.
Added a NEWS.md file to track changes to the package.
power_in_y_axis argument in critical_power() is now reverse_y_axis.
All the previously method_* functions for individual critical power methods have been deleted and critical power should be estimated through the generic critical_power() only.