Futurize for python2/3

Context

This merge request brings compatibility for python 3 while ensuring backwards compatibility with python 2.

Details

The future package has been used to futurize the code. Waves can now depend on python3-xxx instead of python-xxx packages.

If python 2 is to be used instead of python 3, proceed with the usual build and add -DUSE_PY3=OFF to the cmake command line. Note that the additional python-futurize package is required.

Test

Passed on ubuntu18.04 (python 2.7.15 and 3.6.9), debian (gaston, python 2.7), msys2 (python 2.7.16 and 3.8.1)