Clean code_constructions.py

[David Lucas]

As title says.

This file is a huge mess:

• Some import statements are just weird (GCD vs gcd)
• A lot of doctests are a real pain: they cover specific application of theorems, or advanced stuff. Thing is, they were written for Joyner's implementation. Which means I broke a lot of things, and for some of these I'm not even sure how to fix these. See DuadicCodeOddPair, or is_splitting for instance.
• As usual, documentation is poorly written. E.g.

def DuadicCodeOddPair(F, S1, S2):
r"""
    Constructs the "odd pair" of duadic codes associated to the
    "splitting" S1, S2 of n.
"""

• And actually, I'm not even sure some of these methods are useful!
• For the useful ones, consider these as classes would of course be nice...