ep_diffusivity: Calculate the summed diffusivity of an air bubble
In lindseyboulet/fizzTulz: Physiology Analysis and Presentation Helper Functions

Function to calculate the summed diffusivity of air bubble suspended in fluid from the known temperature and viscosity of the system as well as the molecular radii of the bubble. Helper function for bubble lifetime model

1	ep_diffusivity(mol_rad, temp_c = 37, dyn_vis)

`mol_rad`	molecular radius of bubble (use mol_radius function)
`temp_c`	temperature in degrees celcius 37 by default
`dyn_vis`	dynamic viscosity of fluid surrounding bubble (see fizzTulz::dyn_vis)