Add another ViewString method for inverse semigroups

[james-d-mitchell]

This commit introduces a method for ViewString for semigroups satisfying
IsInverseSemigroup and HasGeneratorsOfSemigroup. The rank of the filter
IsSemigroup and HasGeneratorsOfSemigroup is 14, whereas the rank of
IsInverseSemigroup is 21. Previously, if a semigroup learned that it was
inverse but did not have GeneratorsOfInverseSemigroup, then the default
ViewString method for IsInverseSemigroup was used. With this commit the
more elaborate _ViewStringForSemigroups is used instead.

For example, before the change:

gap> S := Semigroup(Transformation([4, 3, 5, 5, 5]),
>                   Transformation([4, 1, 5, 2, 5]), 
>                   Transformation([5, 5, 2, 1, 5]));
<transformation semigroup of degree 5 with 3 generators>
gap> IsInverseSemigroup(S);
true
gap> S;
<inverse semigroup>

After the change:

gap> S := Semigroup(Transformation([4, 3, 5, 5, 5]),
>                   Transformation([4, 1, 5, 2, 5]), 
>                   Transformation([5, 5, 2, 1, 5]));
<transformation semigroup of degree 5 with 3 generators>
gap> IsInverseSemigroup(S);
true
gap> S;
<inverse transformation semigroup of degree 5 with 3 generators>

[olexandr-konovalov]

This method has the same info string as the previous one - could we make it more unique?

[fingolfin]

Houw about "with semigroup generators" and "with inverse semigroup generators"

[olexandr-konovalov]

@james-d-mitchell thanks - looks fine to me except one remark. When ready, I suggest to merge this into the master branch to see if it breaks any tests for GAP packages. If this change will be not disruptive, it could go into stable-4.8 branch too.

[james-d-mitchell]

@alex-konovalov and @fingolfin, good suggestions! I've made the changes.

[olexandr-konovalov]

@james-d-mitchell thanks - looks good to me, I will merge this now. I can't find @fingolfin's comment - apparently it was made on a commit and now gone because of the forced push, but I've looked for it in my email archive to check.