decomposition along the z direction: baroclinic and barotropic components

[BenMql]

In GAFD flows, it would be useful to decompose variables into a barotropic (vertically invariant) and baroclinic (zero vertical mean) components, at least for diagnostics (i.e. when variables are computed in physical space).

[BenMql]

Syntax: in coral.equations, when declaring a quantity in physical space, add baroclinic or barotropic before giving the recipe, e.g.:

>>====================================<<
>>linear_variable_full       :: uFullC <<
>>vertical_decomposition     :: baroclinic <<
>>linear_variable_full_build :: - dy.psi<<
>>====================================<<