straight euclidean distance.  I did this because the kcentroid,
linearly_independent_subset_finder, and krls object's tolerance
parameter is also a measure of squared projection distance and
it is nice if all of these things are the same.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403407