add convergence testing to MCMC

Use acceptance fraction and auto-correlation time to check convergence
of MCMC chain. Option args passed to vmapFit (minAF, maxAF, nEff) set
range for acceptance fraction and effective number of steps (exit once
step number > nEff * auto-correlation time). Based on ()(somewhat
scantydocumentation in emcee and discussion in the Goodman & Weare
paper. Introduces new dependence on acor package.