Fix load_component for label-dict

[noraabiakar]

The cable_component::component property in pyarb returns a cable_cell_variant object which is an alias forstd::variant<arb::morphology, arb::label_dict, arb::decor, arb::cable_cell>.
pybind11 is usually able to present an std::variant object to a python user as the underlying type of the object, but only if there exists a pybind11::class representation of said type.
In this case, pybind11 doesn't have a class representation for arb::label_dict, it has instead a class representation for pyarb::label_dict_proxy. This discrepancy makes it not possible to use load_component to load a label_dict using the pyarb library. This PR fixes the issue.

[thorstenhater]

👍