To avoid weak identification, set sigma to a constant on John Boscardin's advice.
I used roughly the value it seemed to converge to.
Unfortunately autocorrelation remains high.  Perhaps I need to change my jump size,
since acceptance rate of 0.24 is quite low for a one-dimensional problem;
it should be around 0.4
nonlinear: expand step of Metropolis kernel

This attempt to increase the acceptance rate instead lowers it, to
around 0.18.
nonlinear: metropolis 0.8

This increases acceptance rate to 0.29.
nonlinear: Metropolis 0.6

Acceptance rate rises to c 0.37.