Removing sparse matrix for symmetric projection.

[vprusso]

Closes #537

• Removing scipy.sparse in symmetric_projection.py
• Small simplifications and improvements to symmetric_projection.py
• Adding more tests in test_symmetric_projection.py.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.6%. Comparing base (c826b57) to head (6978d20).
Report is 289 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #638   +/-   ##
======================================
  Coverage    97.6%   97.6%           
======================================
  Files         162     162           
  Lines        3212    3216    +4     
  Branches      787     789    +2     
======================================
+ Hits         3138    3142    +4     
  Misses         48      48           
  Partials       26      26           

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[purva-thakre]

LGTM! Thanks for catching the invalid input scenarios.