addressed the comments, improved the code further

- document the arguments of `MakeIsPlistVectorRep`, `MakeIsPlistMatrixRep`,
  in particular the meaning of `check`

- in `MakeIsPlistMatrixRep`, check also `emptyvector`,
  but do *not* check the internal consistency of the vectors in `list`
  (we assume they have been checked when they were created)

- changed argument names in the code:
  use `ncols` instead of `rowlength` or `rl`, use `list` instead of `l`

- use `->` syntax for one-line functions

- set `IsZero` (`IsOne`) in immutable objects created by `ZeroImmutable`
  (`OneImmutable`)

- changed the code such that the mutability of "nested objects"
  fits to the rules for list arithmetic
  (I am not sure whether this is desirable)

- added some tests

lib/matobj.gi

@@ -1177,7 +1177,7 @@ InstallMethod( CopySubMatrix,
 ##
 InstallMethod( TransposedMatImmutable,
  [ IsMatrixOrMatrixObj ],
- TransposedMatMutable );
+ M -> MakeImmutable( TransposedMatMutable( M ) ) );
 
 
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