PCA & TableOfReal: Get fraction variance...

A command to query the selected PCA and TableOfReal object for the explained fraction of the variance if the TableOfReal object were projected onto the PCA space.

## Algorithm

  1. The TableOfReal is converted to a Covariance object.
  2. The Covariance object is projected on the PCA eigenspace and the newly obtained projected Covariance object is queried for the fraction variance.