IsIntersectionOfCongruenceSubgroups should be a category or representation, not a property

[fingolfin]

Right now, IsIntersectionOfCongruenceSubgroups is implemented as a GAP property. But to me, it looks as if this isn't a mathematical property at all, but rather one of internal representation. Both the description of this property, as well as the fact that there are no methods for it, suggest that to me.

In that case, it should probably not be a property at all, but rather a category or representation.

I am actually wondering the same about IsCongruenceSubgroupGammaUpper0 and all the others, but there at least in principle one could imagine that somebody might implement a real method for these to test whether a given group has this property...