Uprank {Copy,Extract}SubMatrix methods for compressed matrices

[fingolfin]

... so that they are actually used.

Also remove redundant CopySubVector methods.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.63%. Comparing base (5fbcf41) to head (1e3f9a6).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6349   +/-   ##
=======================================
  Coverage   78.62%   78.63%           
=======================================
  Files         684      684           
  Lines      292701   292689   -12     
  Branches     8686     8660   -26     
=======================================
+ Hits       230148   230149    +1     
+ Misses      60744    60729   -15     
- Partials     1809     1811    +2     

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[ThomasBreuer]

Yes, the new generic methods that were introduced in #6329 have a higher rank than the old special methods.

Eventually we should perhaps define Is8BitMatrixRep to imply IsMatrix, such that the installations for the different kinds of matrix objects will be more comparable,
but in another pull request.

(According to the documentation of NewRepresentation, such implications are not intended, but they are used for example for IsPlistMatrixRep which implies IsMatrixObj. Thus perhaps the formulations in the documentation of NewRepresentation should be changed.)