nonlinear: switch to Metropolis

Has less than half the run time as before, with not much effect on the
(dismal) effective sample sizes.
There is somewhat arbitrary constant, 1, for the minimum sd of the
metropolis jumps.  I was probably above the bound for this run.
Added acceptance rate to the last column of MCTRACE.
Appears to be getting reasonable numbers.
nonlinear: use ln(prob) for Metropolis

Verified that gives same result as before. and computes same  ratios.
Getting more clever about handling y=0/1 and its probability (just
change z to -z for computations).