Handling of z-coord in multimodel is dodgy

[valeriupredoi]

This block in func:_put_in_cube()

    # no plevs
    if len(template_cube.shape) == 3:
        cspec = [(times, 0), (lats, 1), (lons, 2)]
    # plevs
    elif len(template_cube.shape) == 4:
        plev = template_cube.coord('air_pressure')
        cspec = [(times, 0), (plev, 1), (lats, 2), (lons, 3)]
    elif len(template_cube.shape) == 1:
        cspec = [
            (times, 0),
        ]
    elif len(template_cube.shape) == 2:
        # If you're going to hardwire air_pressure into this,
        # might as well have depth here too.
        plev = template_cube.coord('depth')
        cspec = [
            (times, 0),
            (plev, 1),
        ]

is dodgy as a dodgy plumber.

[valeriupredoi]

'tis mucho better now, after @Peter9192 refactored it 👍