Rename ``normalize``

[SergeStinckwich]

Like separateExpression, normalize on expression has no precise definition (see #399) ... What does that mean to normalize? Apparently this is to simplify formula by moving substraction at the top ... Like separateExpression, this is only used in generateEvents. We have to rename this method with a meaningful name, see what are the specifing needs in generateEvents in order to have a more formal definition at the end.

[mikalziane]

Normalization is only possible when the considered operators, on the considered domains, have appropriate properties.