Covariance computation wants to do a triangular iteration but as a
single loop. Right now it iterates over a square and does nothing half
the time, which is inefficient and has bad worst-case threading
performance. This adds a utility that allows waste-free linear iteration
over a triangle.
Change-Id: I881d5683c65882f87dc2b5f8449a855d22ace755
William Rucklidge