allow resumption of sampling

[sebdalgarno]

I think need to save output using built-in functions and then resume from last iteration while taking advantage of saved warmup config
possibly need new function(analyse_resume() or else can add s3 method for analyse() that works on cmdstan_mcmc_analysis object

[sebdalgarno]

I think for this to work we need to change how we approach sampling/thinning. I see a workflow like:

analysis <- analyse(model, data = data, niters = 1000)
# this thins the samples in analysis$mcmcr
analysis_thin2 <- analyse_thin(analysis, nthin = 2)
# check convergence on thinned
glance(analysis_thin2)

# add 500 more samples
analysis2 <- analyse_resume(analysis, niters = 1000)
# now thin 3 to get 500 final samples
analysis_thin3 <- analyse_thin(analysis, nthin = 3)
# check convergence on thinned
glance(analysis_thin3)