DetectorModel: add helper method to determine if this model is of a certain type

might come in handy for e.g. !632 (merged). Reduces

auto themodel = std::dynamic_pointer_cast<HybridPixelDetectorModel>(model);
if(themodel != nullptr) {
    // ...
}

to

if(model->is<HybridPixelDetectorModel>()) {
    // ...
}