Generalize `initialize_vars` more

[jb-mackay]

Generic initialize - we may be able to unify this with initialize_vars but that would require changing the output type of that (currently a FieldVector). We would probably return a pair of keys, zero_instances where keys are the variable names and the zero_instances are the initialized fields. Then we can package that into a NamedTuple or FieldVector.

Originally posted by @jb-mackay in https://github.com/CliMA/ClimaLSM.jl/pull/51#discussion_r867272597