Add max_nlev to remaining mixing modules

Background, convection, double diffusion, and shear mixing all allocate arrays
in the wrapped coefficient interface based on CVmix_vars%max_nlev rather than
the size of zw_iface.
max_nlev means ddiff and shear don't need zw

This basically rolls back changes to the stand-alone drivers from 760dd99 and
ae3969f -- we don't need to allocate zw_iface (a work-around that was added
when the wrapped interface based the size of new_Mdiff on zw_iface instead of
max_nlev)