Fix `virtual_temperature*()`

The order of the gas constants in a fraction was in reverse, causing the virtual temperature to be lower than the actual temperature, which is wrong. This Merge Request fixes this and adds tests for the virtual_humidity*() functions, asserting that the virtual temperature always increases the temperature iff there is humidity in the air and that the actual value approximately equals the two rule-of-thumbs given at Wikipedia.

Closes #46 (closed)