Allow chain thinning/burnin while running

[richfitz]

This PR allows thinning while the chain runs; this is primarily for cases where the observer will produce a lot of data. To keep the general idea of the chain continuation working, the thinning rate cannot be changed after starting and you can only burnin the initial bit. I think that later we might want to allow "continue but don't append" so that these things can be modified (same with things like changing the proposal)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.46%. Comparing base (070b112) to head (d67dd4d).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #91   +/-   ##
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  Coverage   99.46%   99.46%           
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  Files          69       69           
  Lines        5203     5246   +43     
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+ Hits         5175     5218   +43     
  Misses         28       28           

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