Enable the usage of Manifold with unspecified yTrue (ground truth) parameter.

[kenns29]

Some DS use manifold mainly as a feature segmentation and comparison tool. In that case, they don't necessarily need a ground-truth data and they demand the ability to use Manifold without having to specify the yTrue variable due to the difficulty of obtaining these ground truth data. Considering this use case, it is useful to enable an option to use Manifold without having to specify the yTrue variable, even though this option will make some of the model evaluation functionalities unavailable.

[Firenze11]

This seems reasonable. Assigning it as a P2 and removing the RFC tag.