Removed lil sparse matrix for GHZ state.

[vprusso]

Closes: #536

• Removed sparse.lil_matrix from ghz.py
• Updated tests and docs (no longer need to use .toarray().

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.6%. Comparing base (c6a7f2d) to head (0e4b665).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #637     +/-   ##
========================================
- Coverage    97.6%   97.6%   -0.1%     
========================================
  Files         162     162             
  Lines        3215    3212      -3     
  Branches      787     787             
========================================
- Hits         3141    3138      -3     
  Misses         48      48             
  Partials       26      26             

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[purva-thakre]

LGTM!