Layer thickness function

[jtgasparik]

This PR encompasses 3 function changes summarized below.

1. A function was added to compute the thickness of a single layer in the single particle aerosol representation. Function accepts an aero_phase_idx and returns the thickness that the aerosol phase resides in.
2. A second function (called aero_rep_get_effective_layer_radius__m) was added that computes the radius of a specified layer in an aerosol particle in the single particle representation. This uses the same logic as the function that computes the radius of a particle (aero_rep_get_effective_radius__m in previous versions of this model). However, the code is modified to only computes the radius up to a specified layer. The modal/binned representation returns the radius of the entire particle (since layers do not exist in this representation).
3. Modifications to the function that calculates the radius of a particle. In the single particle representation, this function now calls the aero_rep_get_effective_layer_radius__m function and calculates the radius of the outermost layer. In the modal/binned representation this function returns the radius and remains the same as in previous versions.

[mattldawson]

Looks great! Just one naming change suggestion, but everything else looks good.

[mattldawson]

looks great!