Modify the initialization of H in the double arch geometry model

[DaehoHan]

The previous implementation gives weird PESs, since H at x=-Z is undefined.

def compute_model(q, params, full_id):

    model = params["model"]
    res = Subotnik.double_arch_geometry(q, params, full_id)        

    return res

model_params = {"model":3, "model0":3, "nstates":2}
plot_params.update( { "xlim":[-15.0, 15.0], "ylim":[-0.4, 0.4], "ylim2":[-0.4, 0.4], "show_nac_abs":1 })
dynamics_plotting.plot_surfaces(compute_model, [ model_params ], [0, 1], -15.0, 15.0, 0.05, plot_params)