Ensure coherent interface for factorising polynomials

[albinahlback]

As brought up in #1603, the nmod_poly_factor module seems to append factors instead of setting a new factorisation.

First, we should ensure that the interface of factoring polynomials is coherent throughout FLINT.

Secondly, we should decide on whether we should set or append when factorising. Here, both me and Fredrik thinks that the most intuitive way is to set a new factorisation.

If we decide to append the factors instead, then this needs to be documented.