Added
- `mu_graham()` to calculate viscosity of a gas mixture using mole fraction and viscosity of each component
- `mu_herning()` to calculate viscosity of a gas mixture using molecular weight, mole fraction, and viscosity of each component
Changed
- Renamed `molecular_weight()` function to `mw()`
- Input parameters for `mw_mix()` are now molecular weight and mole fraction of each gas component
Removed
- Deleted the `mu_gas_mix()` function. Use the `mu_graham()` function instead