Maintenance: Apache license headers

[maresb]

I'm doing some maintenance work, auditing the license situation and such. For Apache, it's recommended (but not strictly required) to add a header to all source code files. We should consider doing that here.

Note that @lucianopaz maintains a pre-commit hook for this purpose.

[OriolAbril]

I got some resources about copyright and years. Not sure if out of scope but it seems the current best practice is not including any years in it