fix: enfoce deepcopy for costar to avoid inplace modifications

[oboulant]

closes #162

[codecov]

Codecov Report

Merging #164 (51b0b58) into master (2e6bce6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #164   +/-   ##
=======================================
  Coverage   96.01%   96.02%           
=======================================
  Files          40       40           
  Lines         979      980    +1     
=======================================
+ Hits          940      941    +1     
  Misses         39       39           

Impacted Files	Coverage Δ
src/ruptures/costs/costautoregressive.py	100.00% <100.00%> (ø)

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