Improve terminology in code and output related to 'step', 'sample' and 'iteration'

[mikegilchrist]

Need to fix code and output so that we use the terms, 'step' and 'sample' and avoid 'iteration' (which I believe is synonymous with 'steps'. Could make it clearer by using 'proposal step' and 'sample from chain'

maxGrouping: 22 Starting MCMC Estimate Codon Specific Parameters? TRUE Estimate Hyper Parameters? TRUE Estimate Synthesis rates? TRUE Starting MCMC with 2000 iterations Adapting will stop after 2000 steps Begin saving restart file(s) at sample (iteration): 50 (100)