* Gets the (ρ, φ, θ) spherical coordinates from the current x, y, z cartesian point
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* The algo used is from http://fr.wikipedia.org/wiki/Coordonn%C3%A9es_sph%C3%A9riques#Relation_avec_les_autres_syst.C3.A8mes_de_coordonn.C3.A9es_usuels
*
* @return array an array of 3 floats number, representing the (ρ, φ, θ) spherical coordinates