Add 2D symmetric array checking for `converter` module

[FanwangM]

Also fixed the failing tests because of non-symmetric matrix used. This PR is the final attempt to address #123.

[codecov[bot]]

Codecov Report

Merging #178 (6acba2e) into main (9c74867) will increase coverage by 0.08%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #178      +/-   ##
==========================================
+ Coverage   85.49%   85.57%   +0.08%     
==========================================
  Files          13       13              
  Lines        1020     1026       +6     
==========================================
+ Hits          872      878       +6     
  Misses        148      148              

Files	Coverage Δ
DiverseSelector/converter.py	98.87% <100.00%> (+0.03%)	:arrow_up:
DiverseSelector/tests/test_converter.py	100.00% <100.00%> (ø)