message: Move assert for expression model further up
  The expression model is already used there, so moving the assert before that.
  We should probably even throw, so that people don't stumble upon this in Release mode.
diff: @@ -440,6 +440,7 @@ inline std::pair<core::Mesh, fitting::RenderingParameters> fit_shape_and_pose(
  // Current mesh - either from the given coefficients, or the mean:
  VectorXf current_pca_shape = morphable_model.get_shape_model().draw_sample(pca_shape_coefficients);
  + assert(morphable_model.has_separate_expression_model()); // Note: We could also just skip the expression fitting in this case.
  // Note we don't check whether the shape and expression model dimensions match.
  VectorXf current_combined_shape =
  current_pca_shape +
  @@ -478,7 +479,6 @@ inline std::pair<core::Mesh, fitting::RenderingParameters> fit_shape_and_pose(
  fitting::RenderingParameters rendering_params(current_pose, image_width, image_height);
  const Eigen::Matrix<float, 3, 4> affine_from_ortho = fitting::get_3x4_affine_camera_matrix(rendering_params, image_width, image_height);
  - assert(morphable_model.has_separate_expression_model()); // Note: We could also just skip the expression fitting in this case.
  expression_coefficients = fit_expressions(morphable_model.get_expression_model().value(), current_pca_shape, affine_from_ortho, image_points, vertex_indices);
  // Mesh with same PCA coeffs as before, but new expression fit (this is relevant if no initial blendshape coeffs have been given):
  