Add property field for refractive index n

[kdreher]

Please describe your :rocket: feature request. After an mcx update that allowed the 3D definition of mua/mus/g/n as float values, we want to include this option as well.

Describe the solution you'd like Adding the refractive index as property field for each tissue type and adding this as well as the existing g maps to the input of mcx should suffice.