Simplify embedding network

A MR that tries to simplify the embedding network.

• Remove the regime: ["hnm", "rp"], we always use that anyway so just enforce it.
• Delete other regimes such as "low_purity" or "query_all_points"
• Instead of the remaining "norm" regime, define normalize_output variable
• Remove bidir so that only unidirectional graphs are considered.
  • Bidirectional graphs might not even work
  • Become difficult to maintain
  • At this point, if we would really want to use bidirectional graph, might be better to define a separate class

2 other changes that I could happily revert:

• Rename nb_layer to n_layers
• Rename emb_hidden to n_hiddens

It is possible this improve readability since that's the convention I've using throughout the repository. HOWEVER, this involves changing this in ALL THE PIPELINE CONFIGURATIONS (hence a lot of changes!)

@fgiasemi Let me know your thoughts