Add water tracers to PUMAS microphysics scheme

Add water tracer functionality to the PUMAS cloud microphysics scheme, and ensure that it properly conserves the ratio of the water tracer relative to standard water. Note that _in theory_ the water tracing component can be done outside of PUMAS itself (but discussions will need to happen to determine if this is the optimal design choice).

This can be validated by creating/modifying a PUMAS snapshot test with a water tracer with a known ratio (e.g. 50%) relative to standard water, and ensuring that the ratio is conserved after the full scheme has been run.