Enhancements to modified Beer Lambert Law

[rob-luke]

Describe the new feature or enhancement

In the discussion at #8711 the following enhancements for the BLL were mentioned. I am listing them here so these suggestions don't get lost.

• Add support for alternative extinction coefficients.
• Refactor code to have private function where source-detector distance can be specified.
• Add some references to literature.
• Add age dependent partial path length factor (reference)

Additional comments

The code works fine as is. This is just optional improvements for when anyone has time. Please list additional feature requests below too.

[scholzri]

Hi! I would like to suggest a new feature to this function. It should be possible to use a ppf per wavelength, similar to what is possible in Homer3 where ppf is a vector of factors per wavelength.

see https://github.com/BUNPC/Homer3/blob/master/FuncRegistry/UserFunctions/hmrR_OD2Conc.m

[larsoner]

@AdmerStroh would you be up for adding support for this to MNE-Python?