This PR adds the method sylow_subgroup to the PermutationGroup class for computing Sylow subgroups. As part of this:
Implemented orbit and block system action homomorphisms (orbit_homomorphism and block_homomorphism in the homomorphisms module).
Added the method minimal_blocks for returning all minimal block systems of a group
Fixed a bug related to the _strong_gens_slp attribute: it used to be set inside schreier_sims_incremental however this method is called by normalizer and stabilizer methods which led to a key mismatch with respect to strong_gens.