numerical derivative step size is precision dependent

[pkienzle]

Need to use eps=sqrt(1e-16) for double precision and sqrt(1e-8) for single precision when computing step size h for numerical differentiation. If the step size is too small, the cancellation error in f(x+h) - f(x) dominates, and if the step size is too large, the numerical derivative error dominates. Bumps mostly assumes double precision.

[butler]

As agreed on fortnightly call of June 21, am moving all minor priority tickets to next release (+1) given the resources and time available to get this release out.