Add Partial trace to density matrix

[itsubaki]

No description provided.

[codecov]

Codecov Report

Merging #18 (6a8c292) into main (bd72e34) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #18   +/-   ##
=======================================
  Coverage   99.77%   99.77%           
=======================================
  Files          17       17           
  Lines        1330     1348   +18     
=======================================
+ Hits         1327     1345   +18     
  Misses          2        2           
  Partials        1        1           

Impacted Files	Coverage Δ
pkg/quantum/density/matrix.go	100.00% <100.00%> (ø)

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