Add endpoint for carbonation data

[stopthatcow]

This should report both tan, keg pressure, and dissolved co2 volumes based on this formula:
V = (P + 14.695)_(0.0181 + 0.090115_exp( (32-T)/43.11) ) - 0.00334
where v is volumes of co2, T is them in deg F and P is pressure in PSI