Add license to header files

I recently learned a few things about open-source licensing and realized we should add a license header to the files in src/pystencils/include. Since a user might use pystencils to generate a kernel, save it as a .cpp file, and copy that into another project (waLBerla, ESPResSo, ...), they might also need to copy these header files along with it. The generated code doesn't have or need a license header (it is not an original work), but the non-generated header files should have one so their origin and usage permissions are clear.

While we're at it, let's relicense these header files to BSD-3. Right now they are covered by pystencils' AGPLv3 license, but that can theoretically be a problem even for incorporating into GPLv3-licensed projects. I am the sole copyright holder almost everything in these files, with just a few small exceptions according to their Git history:

• The original version of philox_rand.h (6a01f3e2) consisted of code that @bauer extracted from https://github.com/DEShawResearch/random123. That actually was BSD-3-licensed already and should thus have included a notice all along.
• PyStencilsField.h is not used anymore and can be deleted.
• half_precision.h and gpu_defines.h created and only ever edited by @holzer. So we need @holzer's permission to relicense these files.