improve speed of near field computation

[ronandrevon]

By implementing spherical harmonics using the legendre polynomials recurrence formula, the speed of the wave function computation should be improved.

[kitine]

Implementation of the spherical harmonics :

Recurrence formula used for the implementation :

Explenation of the implemented matrix

Implementation of the legendre polynomes

Formula used to implement spherical harmonics

Implementation of the spherical harmonics

[kitine]

Verification of the implementation of the spherical harmonics :

[kitine]

Speed test

For the computation of spherical harmonics with l=10, m=[-10;10]
Using the recurrence formula : 0.004826784133911133 s
Using python's function : 0.16061711311340332 s

For the computation of the total field
Using the recurrence formula : 20.19532084465027 s
Using python's function : 25.091874837875366 s