bdrate -> reworked with pchip

bdrate metric curve fitting sometimes resulted in strange curves
that run completely outside the bounds of the interpolated
points and produces poor measures of graph quality.   This
uses pchip Piecewise Cubic Hermite Interpolating Polynomial
and sample evaluations to avoid the problem

Change-Id: I54c95c39d0696bf433844063c211af0f4430d607
1 file changed