Truncate retrieved arrays to chain length

[michaelosthege]

Closes #38

[codecov-commenter]

Codecov Report

Merging #39 (0d94d5c) into main (249acd6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #39   +/-   ##
=======================================
  Coverage   95.51%   95.52%           
=======================================
  Files           8        8           
  Lines         558      559    +1     
=======================================
+ Hits          533      534    +1     
  Misses         25       25           

Impacted Files	Coverage Δ
mcbackend/__init__.py	71.42% <100.00%> (ø)
mcbackend/core.py	85.47% <100.00%> (+0.12%)	⬆️

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