Remove is_air() and convert its users to gasmix

Also make gasname() and get_gas_string() global functions (which allows us
to delete code elsewhere).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Convert plot_gas_value to gasmix

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Rename two C++ methods to the more typical CamelCase

Silly little change. Whatever.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>