Add gas viscosity function for inorganic compounds

[wigging]

Include data file from Yaws' database. Add function to calculate gas viscosity for inorganic compounds using coefficients in the data file.

[wigging]

Added functions mu_gas_inorganic and mu_gas_organic to gas_viscosity module. Still need to do the following:

• [x] write tests
• [x] update documentation

[wigging]

Done.