Chemics v19.6

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