Include dependencies via git-submodules

This merge request adds aocommon as a submodule to EveryBeam and replaces the hard-copies of the pybind11 and eigen3 by submodules.

Please note: we keep the submodules at a fixed commit/tag, determined by the superproject. By doing so we avoid silent updates of the dependencies.

Merge request closes #15 (closed)

@dijkema : you'll find this interesting.