Implement a function for creating a generic tissue from optical parameters

[RecurvedBow]

Please describe your :rocket: feature request. As a user with access to custom data about optical parameters, I would like to define a tissue from these optical parameters instead of using the predefined tissue compositions in the tissue library. While TissueLibrary.constant(...) exists, it does not work if my optical parameters are different depending on the wavelength.

Describe the solution you'd like Add a function get_generic_tissue in the tissue library returning the correct tissue composition from the optical properties mu_a, mu_s, g which are defined as a spectrum.

Describe alternatives you've considered

Additional context TissueLibrary.constant(...) should be refactored into a special case of TissueLibrary.get_generic_tissue(...).