Add O2, N2, and Ar

[jbeaulie]

While CO2, CH4, and N2O are frequently the focus of GC-based dissolved gas measurements, investigators may also be interested in O2, N2, and Ar, which can be measured via GC-TCD. Consider adding functions for these gases.

There may also be interest in calculations for gases typically used in gas-exchange measurements, such as SF6 and propane. Consider providing functions for these gases.

[kcawley]

We are in the process of putting a package together for reaeration which will include SF6 and propane. It's a good idea to try to use these functions as part of that package.