Version 7.01

Abstract class StackModel was created to give dielectric_stack the properties of a Model. Specific classes No_StackModel, Stack_StackModel, SingleFilm_StackModel, DoubleFilm_StackModel, and GradedFilm_StackModel implement specific film stack models. These models inherit Model, so they make it easier to parametrically vary parameters associated with a film stack. All scattering models that allowed for a dielectric_stack have been upgraded to use StackModel instead.

Added Legendre_Phase_Function to the list of classes inheriting Phase_Function.

Corrected some errors in the thread safety using mutexes.

Corrected some errors in coordinate systems for polarization in BRDF_Model.