type-hint for `fmin` in `colorednoise.powerlaw_psd_gaussian`

[charlesbmi]

pyright was throwing an error for me in my calling code, because it thought powerlaw_psd_gaussian expected fmin to be an int (based on the fmin=0 function signature). This updates fmin to explicitly be a float value, as noted in the docs.

[felixpatzelt]

@charlesincharge thank you for the PR. I opened a followup one to complete the type annotations to all parameters. If you have time, please check if this works for you. I will then publish the new version to PyPI.

[charlesbmi]

Works well for me, thanks @felixpatzelt !