fixed_dz_advection: include in code | remove from namelist

[bertjebertjek]

The fixed_dz_advection flag in the namelist is confusing for all but the most experienced users. For certain (most?) applications it should be set to true to produce realistic behaviour, after Dylans modifications of the advection code. Therefore I suggest we hardcode the desired behaviour into either the advection or wind code and remove the flag from the namelist.

Expected Behavior

for anything but linear theory in the wind settings, fixed_dz_advection should be TRUE. (but @gutmann can confirm this).

[gutmann]

@bertjebertjek is this done now? If not, can you implement exactly this (hard code the desired behavior) It may be simplest at first to hard code the setting for fixed_dz_advection based on the selected wind scheme in the options object.