DetectorField: only retrieve pixel index when necessary

    template <typename T, size_t N>

    T DetectorField<T, N>::get(const ROOT::Math::XYZPoint& pos, const bool extrapolate_z) const {

        // Return empty field if outside the matrix or no field is set

        auto [px, py] = model_->getPixelIndex(pos);

        if(type_ == FieldType::NONE || !model_->isWithinMatrix(px, py)) {

        // Return empty field if no field is set

        if(type_ == FieldType::NONE) {

            return {};

        }

        // Return empty field if outside the matrix

        if(!model_->isWithinMatrix(pos)) {

            return {};

        }

        // For per-pixel fields, resort to getRelativeTo with current pixel as reference:

        if(mapping_ != FieldMapping::SENSOR) {

            // Calculate center of current pixel from index as reference point:

            auto [px, py] = model_->getPixelIndex(pos);

            auto ref = static_cast<ROOT::Math::XYPoint>(model_->getPixelCenter(px, py));

            // Get field relative to pixel center: