Reduce method definitions for cat of AbstractQs

[dkarrasch]

This is related to #1233. The remaining function definitions are necessary because the generic counterparts treat everything not-an-array as a scalar, so we absolutely have to teach it to treat AbstractQs as arrays in concatenation. In the assignment of a subarray this even allows to avoid a temporary array.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.92%. Comparing base (189d49d) to head (ac37ab6).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1500      +/-   ##
==========================================
- Coverage   93.93%   93.92%   -0.01%     
==========================================
  Files          34       34              
  Lines       15982    15980       -2     
==========================================
- Hits        15012    15010       -2     
  Misses        970      970              

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