Add affine matrix decomposition function to extract translation, rotation and scaling consistently

[rhugonnet]

In #502 we add functions to extract translations/rotations, but this only works for a rigid transformation.
For an affine, transformation, we need to decompose the scaling/shear too.
See for instance: https://math.stackexchange.com/questions/237369/given-this-transformation-matrix-how-do-i-decompose-it-into-translation-rotati

Additionally, having these functions as methods of AffineCoreg is not the most easy for manipulation (cannot use these methods to build matrixes for example in the documentation, or for the tests, for instance).
Should we have separate public functions in affine.py instead?