CREATEMEDIUM Creates a vmcmedium structure

 USAGE:

       vmcmedium = createMedium(vmcmesh);

          returns

                   vmcmedium.refractive_index(:) = 1;
                   vmcmedium.scattering_coefficient(:) = 0;
                   vmcmedium.absorption_coefficient(:) = 0;
                   vmcmedium.scattering_anisotropy(:) = 0;

       vmcmedium_out = createMedium(vmcmesh, vmcmedium);

           repeats the entries in vmcmedium so that the size of each array
           is equal to the number of elements in the mesh.

 DESCRIPTION:

       The purpose of this function is to create or resize the arrays in
       vmcmedium so that they match the number of elements in the mesh.

 INPUT:

       vmcmesh       -  mesh structure, contains the geometry of the
       system

 OPTIONAL INPUT:

       vmcmedium     -  medium structure, used to set optical
                        coefficients

 OUTPUT:

       vmcmedium_out -  formatted medium structure

 SEE ALSO:

       https://inverselight.github.io/ValoMC/structures.html

 This function is part of ValoMC toolbox