ODEExporter: use get_model_dir if `outdir` is None

[dweindl]

Use consistent default model directories everywhere.

[dweindl]

Already used in amici.exporters.sundials.de_export.DEExporter, only amici.jax.ode_export.ODEExporter uses a different default path.
@FFroehlich : You choose. As the jax models have a separate model-version, it might make sense to include this in the default path instead of the amici version.