add_marbl (compute): Better [vertical] interpolation routines

[Michael Levy]

Keith has pointed out that the routines I'm using to interpolate initial conditions and iron flux are deprecated. I need to dig in to see exactly which functions I'm using, and which ones they've been replaced by

[Michael Levy]

I think the iron flux is okay, but I'm using tracer_Z_init() for tracers instead of MOM_initialize_tracer_from_Z()

[Michael Levy]

Fixed in becd34f (March 2023)