Hook up slconstr.gi code, possibly move it to blackbox / "generic" directory

[fingolfin]

The file slconstr.gi contains, according to a comment, "Peter Brooksbank's code for constructive SL and PSL recognition."

However, it is not hooked up, and hence never called.(With the exception of SLCR.FindHom, which is potentially called by FindHomMethodsProjective.ThreeLargeElOrders, though in practice our current test suite never runs into that case.

[fingolfin]

This genuine is genuinely useful and we should indeed make use of it eventually.