Update schaapcommon and add gcc-6_3 docker file
This is necessary to fix compilation on gcc 6, which is used by Matthijs vdW. The old schaapcommon gives errors like
error: enclosing class of constexpr non-static member function 'schaapcommon::facets::Vertex schaapcommon::facets::BoundingBox::Min() const' is not a literal type
I've also added a Dockerfile for gcc 6.3.
Edited by Andre Offringa