Improvements to sample_chain interface

[matt-graham]

Some changes to improve user experience of sample_chain function and decrease side effects

• Copy initial state in sample_chain to avoid mutating argument.
• Change how adapters are initialize to no longer require passing in proposal, with this instead dealt with internally within sample_chain by passing in proposal to initialize, update and finalize method. This makes it easier to resuse adapters across proposals.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (04ef350) to head (05cbe20).
Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #43   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          391       398    +7     
=========================================
+ Hits           391       398    +7     

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