generalization to support multiple C systems

[matt-long]

We are interested in a capacity to simulate multiple DIC/Alk tracers simultaneously within MARBL, each with different forcing representative of CDR.

An approach to doing this would generalize the machinery supporting the ALT_CO2 tracers and enable something like DIC_{n} and Alk_{n} tracers.

@dafyddstephenson is planning to scope this out and we'd love feedback on the requirements for an effective implementation. We'd plan to maintain backwards compatibility (i.e., preserve the capacity to support the ALT_CO2 tracer names)