Document different behaviours of landmark mappings file better

[patrikhuber]

See this code section here:

eos/include/eos/fitting/fitting.hpp

Lines 400 to 420 in 9ac3101

	// If the MorphableModel does not contain landmark definitions, we expect the user to have given us
	// direct mappings (e.g. directly from ibug identifiers to vertex ids). If the model does contain
	// landmark definitions, we expect the user to use mappings from their landmark identifiers (e.g.
	// ibug) to the landmark definitions, and not to vertex indices.
	// Todo: This might be worth mentioning in the function documentation of fit_shape_and_pose.
	int vertex_idx;
	if (morphable_model.get_landmark_definitions())
	{
	const auto found_vertex_idx =
	morphable_model.get_landmark_definitions().value().find(converted_name.value());
	if (found_vertex_idx != std::end(morphable_model.get_landmark_definitions().value()))
	{
	vertex_idx = found_vertex_idx->second;
	} else
	{
	continue;
	}
	} else
	{
	vertex_idx = std::stoi(converted_name.value());
	}

I am not sure whether this is documented somewhere, I'd have to check. If it isn't already, would be worth documenting this more prominently.