GeoManager: re-add material key check for non-GDML passive elements

    for(auto& passive_element : passive_elements_) {

        passive_orientations_[passive_element.getName()] = calculate_orientation(passive_element);

        // Check for mandatory but herein unused keys:

        // Check for mandatory but hitherto unused keys:

        auto check_key = [&](const Configuration& cfg, const std::string& key) {

            if(!cfg.has(key)) {

                throw MissingKeyError(key, cfg.getName());

            }

        };

        // Check type keyword

        check_key(passive_element, "type");

        // Check material unless it's a GDML file placement

        auto type = passive_element.get<std::string>("type");

        std::transform(type.begin(), type.end(), type.begin(), ::tolower);

        if(type != "gdml") {

            check_key(passive_element, "material");

        }

    }

    // Load the list of standard model paths