AmpGen update and models for Xi spectroscopy

The AmpGen update consists of a fix for the Canonical/Helicity formalism with deep decay topologies (adapted to c++11 from this commit on github) and the implementation of the P-vector formalism for generic K matrices (see this PR).

Models for Xi spectroscopy from charm decays will be added. Namely \Lambda_c^+\to\Xi^-K^+\pi^+, \Xi_c^+\to\Xi^-\pi^+\pi^+ and \Xi_c^+\to\Xi^-K^+\pi^+.

TODO:

• cross check of AmpGen versions with lib_diff
• validate generator level output
• add \Xi_c^+\to\Xi^-\pi^+\pi^+
• add \Lambda_c^+\to\Xi^-K^+\pi^+

/cc @tevans