Replacing "pure_to_mixed" with "vector_to_density_matrix" and then renaming to "to_density_matrix"

[Shivansh20128]

Description

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The following changes are made as a response to the issue #847

1. Remove the pure_to_mixed function (along with the associated tests)
2. Replace any usages of pure_to_mixed with vector_to_density_matrix.
3. Rename vector_to_density_matrix to just to_density_matrix as the input does not necessarily need to be a vector .

#Resolves #847

Changes

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• Change 1

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.8%. Comparing base (58bf855) to head (c92c661).
Report is 16 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #853     +/-   ##
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- Coverage    97.8%   97.8%   -0.1%     
========================================
  Files         168     167      -1     
  Lines        3259    3251      -8     
  Branches      717     715      -2     
========================================
- Hits         3189    3181      -8     
  Misses         46      46             
  Partials       24      24             

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[vprusso]

This LGTM! Perhaps we wait for @purva-thakre to give her review as well.

Nice work, @Shivansh20128 !