Make orthopoly_recur(legendre_p, ...) indices consistent

The recurrence relation given by orthopoly_recur() for legendre_{p,q}() is inconsistent with relations for other polynomials in that it gives the Legendre polynomial of degree (n) in terms of polynomials of degree (n-1) and (n-2) instead of giving the (n+1) polynomial in terms of (n) and (n-1) like the others. This patch makes it consistent with the other results (such as assoc_legendre_{p,q}) and with A&S p782.

This patch also corrects two recurrence relations (ultraspherical and hermite) in the comments. orthopoly_recur() gives the same relations as A&S, but there were sign errors in the comments.

To avoid possibly garbled text, I'll also attach a file showing the previous result for legendre_p(n,x), the result for assoc_legendre_p(n,0,x) and the new result for legendre_p(n,x), showing that the latter two are in the same form. (I guess I can upload other files after I initially submit this).

Thanks,
Kris Katterjohn