Fixing PPT and other misc.

[vprusso]

• Updating PPT code to use picos over cvxopt/cvxpy.
• Fixing bug in calculate_vector_matrix_dimension
• Adding vector_to_density_matrix function
• Updating prior usage of pure_to_mixed to vector_to_density_matrix

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bfeae4f) 98.1% compared to head (c193430) 98.1%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #469   +/-   ##
======================================
  Coverage    98.1%   98.1%           
======================================
  Files         160     161    +1     
  Lines        3108    3096   -12     
  Branches      749     753    +4     
======================================
- Hits         3049    3038   -11     
  Misses         37      37           
+ Partials       22      21    -1     

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[purva-thakre]

Except for some minor comments to tests, this PR is almost there!

You might see some comments related to toqito/state_opt/ppt_distinguishability.py later. I don't have the time to do a deep dive right now. I want to understand how the new changes are not flagged by pytest as uncovered by tests.

[vprusso]

@purva-thakre changes made. If they are good with you, feel free to lift the "changes requested" flag and I'll merge away. Thank you again!