change mcmc.control names?

I still find some of the options controls a little unwieldy (run.opts, mcmc.opts, etc.); I'm not sure about the naming convention (dot.separated vs camelCase), and at least within mcmc.opts, the names of the arguments (mcmcstep, mcmcpars etc.) are redundant -- they could be shortened to (step, pars, ...) -- although perhaps I should double up for now (i.e. have both a 'step' and an 'mcmcstep' optional parameter, and warn if the 'mcmc*' version is used)?

Start by reviewing the existing run.opts and mcmc.opts option names, including arguments such as mcmcstep and mcmcpars. Clarify the naming convention and compatibility approach; the work is done when the API naming and any transition behavior are agreed and consistently applied.

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