MAINT: Unit tests, complete documentation for EisensteinHuHalofit class

[jucordero]

Description

#565 Does not include unit tests for EisensteinHuHalofit and has incomplete documentation for some of the class methods
Unit tests can either compare to existing precomputed values obtained with Halofit on a known linear power spectrum, or obtained from external sources

• Write unit tests for EisensteinHuHalofit
• Finish docstrings for EisensteinHuHalofit and the rest of the PowerSpectrum sub classes