improvements to assessing curve fit (r2 and rmsd)

[jbloom]

Improvements to metrics for assessing curve fit (see here):

• The coefficient of determination (r2) now is one if all points are fit by a straight line, rather than engative infinity.
• A root-mean-square-deviation (square root of mean residual) is now calculated as the rmsd attribute of HillCurve objects and reported in fit parameter summaries from CurveFits.