Enable early termination

[nefrathenrici]

This PR changes update_ensemble to return the termination flag passed by EKP, which is in turn checked with !isnothing(terminate).

Closes #104

[codecov]

Codecov Report

Attention: Patch coverage is 43.75000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 27.42%. Comparing base (d4b6acc) to head (476102e).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/backends.jl	35.71%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
+ Coverage   27.03%   27.42%   +0.39%     
==========================================
  Files           8        8              
  Lines         344      350       +6     
==========================================
+ Hits           93       96       +3     
- Misses        251      254       +3     

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