Changes to BlockArray and gradient operators

[bwohlberg]

• Modify non-Cartesian coordinate gradient operators to output Array instead of BlockArray.
• Add stack method to BlockArray

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.65%. Comparing base (756c550) to head (0af1ebc).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #591      +/-   ##
==========================================
+ Coverage   93.53%   93.65%   +0.12%     
==========================================
  Files          92       92              
  Lines        6181     6185       +4     
==========================================
+ Hits         5781     5792      +11     
+ Misses        400      393       -7     

Flag	Coverage Δ
unittests	93.65% <100.00%> (+0.12%)	⬆️

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