Added potential fCO2 and pCO2 (fCO2pot, pCO2pot) as requested

Jean-Pierre,

This is a first try to follow up on our conversation a couple of weeks ago, which concluded that we should add fCO2pot and pCO2pot to carb.R.  I think this modified carb.R works, based on a few quick tests. I'll know more in a few days once I compare these new seacarb results to mocsy.  While programming, I found that it was necessary to add a new option "gas" (as an input argument). That can be either 'potential' (the default) or 'insitu'.  It is required because for flags 21 to 25, pCO2 is an input variable and carb.R needs to know if that is potential pCO2 or in situ pCO2 in order to treat it correctly. Conversely, for the other flags, pCO2 and fCO2 are calculated so we don't need the 'gas' option.  Perhaps you could think of a better name for this option.

Cheers,

Jim