Use cecill license

Replace MIT license with CeCILL-C (as in the original repository).

I have two separate commits (for easier review):

1. Push LICENSE files (both in French and in English, as recommanded by them)
2. Propagate the License on all Python files using https://github.com/google/addlicense

Should I also put the license on the documentation files? other ones?

dan/datasets/analyze/__init__.py

+# Copyright Teklia (contact@teklia.com) & Denis Coquenet
+# This code is licensed under CeCILL-C
+
 # -*- coding: utf-8 -*-
 """
 Analyze dataset and display statistics in markdown format.