scipy.stats.distributions integration

[jorenham]

• add methods l_moment, l_stats, etc. to the "generic" and "frozen" distn bases (monkeypatch)
  • rv_continuous (depends Population (T)L-moments from CDF and PPF #6), and use Implement the known theoretical distribution L-moments #5 + the name attribute to check if specific theoretical L-moments are known
  • rv_discrete (depends Population L-moments of discrete univariate distributions #9)
• for the theoretically-known distributions from Implement the known theoretical distribution L-moments #5, override l_moment on the specific rv_continuous instances