enable CSC diagonal counting for triu/tril

[goulart-paul]

Modifies KKT assembly internals so that the function that counts diagonal entries works for either triu/tril form, but the form of the matrix must be explicitly passed.

This function is currently only ever applied to the user-supplied P matrix, so should only ever count diagonal terms on a triu matrix. Including this modification anyway since it makes assumptions more explicit, and might also prevent a bug in some future version that allows full matrices to be passed. Also added some unit tests.

Related : #143, which this replaces.

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.10%. Comparing base (7036899) to head (3161be3).
Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/algebra/csc/utils.rs	83.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #145      +/-   ##
===========================================
- Coverage    88.14%   88.10%   -0.04%     
===========================================
  Files           77       77              
  Lines         6189     6196       +7     
===========================================
+ Hits          5455     5459       +4     
- Misses         734      737       +3     

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