make checksquare fail on non-matrices

[araujoms]

Closes #1362

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.89%. Comparing base (7e11b5e) to head (ff2c014).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1476      +/-   ##
==========================================
- Coverage   93.89%   93.89%   -0.01%     
==========================================
  Files          34       34              
  Lines       15933    15929       -4     
==========================================
- Hits        14961    14957       -4     
  Misses        972      972              

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