Clean up sample loop, pass discard_sample keyword as kwarg to step/step_warmup

[penelopeysm]

This PR:

1. Fixes Num_warmup / discard_initial / thinning error (?!) #196.

2. Fixes Pass information through to step / step_warmup about whether a step will be discarded #185.

3. Abstracts step_warmup and step into a single function, so that this pattern doesn't have to be repeated a few times inside sample().

if i < num_warmup
   step_warmup(...)
else
   step(...)
end

4. Same with saving samples and invoking the callback. Those two things should only ever be called in tandem and if there isn't a single function for them, it just makes it too easy to accidentally do one but not the other. Also this code is just cleaner.

[github-actions]

AbstractMCMC.jl documentation for PR #194 is available at:
https://TuringLang.github.io/AbstractMCMC.jl/previews/PR194/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (a346c51) to head (8e7457d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #194   +/-   ##
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