GluingPerms claims to be generic, but is not

[baburton]

Specifically, it cannot be instantiated for non-standard dimensions because it relies on Perm<dim+1>::Sn_1. This is not an urgent problem, but any fix should be done carefully since (presumably) it will have to change to the lexicographical ordering described by Perm::index() and Perm::order(), which is not the same ordering as used by Perm::Sn_1.