Change license to BSD 3-clause

As discussed in earlier weekly meetings (see action points from week02/2018 till week09/2018), the prospective re-use of BEAT core components to re-implement some Bob frameworks would require a shift in the licensing terms for some core BEAT packages.

@samuel.gaist: could you please do that before the release? The idea is to go from AGPLv3 to BSD 3-clause (as the rest of Bob) for all packages up to beat.cmdline. We exclude beat.web from this, leaving it as AGPLv3.

We should do the following, in details:

1. Remove the file LICENSE.AGPL
2. Create a new file called LICENSE the contains the stock BSD 3-clause text (see info here: https://gitlab.idiap.ch/bob/bob.admin/tree/master/templates)
3. Modify the file MANIFEST.in to include LICENSE instead of LICENSE.AGPL
4. Modify the file setup.py to list the license correctly on the classifier list (replace license key by License :: OSI Approved :: BSD License)
5. Modify the file conda/meta.yaml to set the license appropriately. Should read license: BSD 3-Clause and license_family: BSD on the about section, replacing the current AGPL stuff.
6. Double-check all headers from .py files - @flavio.tarsetti may have a script to do this.

Please consult with @philip.abbet since I think rechecked, by then, the feasibility of this move and found no issues against doing this.