Replace c++17 std:: implementation of legendre poly by boost
assoc_legendre is not in c++11, and we still need EveryBeam to build there.
This fix should be checked, because std and boost have a different implementation. std:: doesn't include the -1^m phase term, so I've removed this. However, confusingly it seems that phase term was only applied when m < -1... Is this correct? (@tol ?)