BUG: `schechter_smf` callable input and docs

[philipp128]

Description

This PR enables alpha and m_star in schechter_smf being functions. Before it was prevented by the schechter_smf_mass function. To resolve I added a set of decorators to schechter_smf_mass. Closes #523.

Furthermore, I included schechter_smf in the docs where they weren't included before. Closes #522.

Checklist

• [x] Follow the Contributor Guidelines
• [x] Write unit tests
• [x] Write documentation strings
• [x] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request

[philipp128]

I made the requested changes. I also added a sentence in the docs of alpha that it must return a scalar value if defined as a callable.