BUG: corrected behavior of assoc. Legendre fct. of 2nd kind across unit circle

This PR attempts to address the issues raised in gh-2840 by implementing two versions of the associated Legendre function of the second kind. Type 2 corresponds to a cut at |z|>1 while type 3 (default) corresponds to a cut at |z|<1. The behavior is consistent with the associated Legende function of the first kind implemented earlier and the values for the type variable correspond to the choice of mpmath (although in mpmath type=2 is the default). A few tests have been added, but probably test_mpmath also needs to be somewhat adapted.