Legendre defines Legendre Polynomials based on of Alan Jeffrey's
Handbook of Mathematical Formulas an Integrals. Please see page 286-288.
Information also obtained from: http://mathworld.wolfram.com/LegendrePolynomial.html
This code is based on the Open Source Physics class for Hermite polynomials.
Gets the nth Legendre polynomial. If it has already been calculated
it just returns it from the list. If we have not calculated it uses
the recursion relationship to calculate based off of the prior