Add water tracers to zhang_mcfarlane deep convection scheme

Add water tracer functionality to Zhang-Mcfarlane (ZM) deep convection scheme, and ensure that it properly conserves the ratio of the water tracer relative to standard water.

This can be validated by creating/modifying a ZM 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.