Check Gaunt coefficients and fix if needed

[certik]

Reported by László Oroszlány and Laszlo Szunyogh.

There is a small mistake (or just rather an inconsistency) in the way you define the c-type Gaunt coefficients in the section about Legendre Polynomials. There is a factor 4_k+1 that should be 2_k+1 to make it consistent with the definition trough the wigner 3-j-s. you simplify by (2_k+1)/(4_k+1) at some point..

[certik]

Indeed. In the section http://theoretical-physics.net/dev/src/math/spherical-harmonics.html#id6, the last equation should have 2k+1 in the denominator instead of 4k+1, only then it corresponds to Condon & Shortley.