Add symmetry or pseudo-symmetry property to some functions

[pedritomelenas]

gap> l:=IrreducibleNumericalSemigroupsWithFrobeniusNumber(15);
[ <Numerical semigroup>, <Numerical semigroup>, <Numerical semigroup>, <Numerical semigroup>,
  <Numerical semigroup>, <Numerical semigroup>, <Numerical semigroup> ]
gap> List(l, HasIsSymmetric);
[ false, false, false, false, false, false, false ]
gap> List(l, HasIsIrreducibleNumericalSemigroup);
[ true, true, true, true, true, true, true ]