Clean in ymport.stl

Remove unneded and commented lines
Remove service-messages in CMakeLists
Add missed #pragma once
Add RELEASE file.
Update changelog.
Delete RELEASE file.
Revert "Revert "Set minimal required Eigen3 version 3.2.1"" / Thanks Anton for uploading the backport

This reverts commit 56bb96e9d50ac2e803df5a7003388696ff95c43a.
fixed bug in export.VTKExporter.exportInteractions (question #250922)
modification of utils.UnstructuredMesh, added utils.polyhedron
Add YADE_PTR_DYN_CAST to define dynamic_cast.
Remove all "using namespace std;"
Fix compilation with disabled openmp.
Roll back last 2 commits.
Remove most of "using namespace std;"
Replace math.h by cmath in includes.
Remove some more "using namespace std;"
Remove "using namespace std;" everywhere. Tests are failing.
Replace abs by std::abs.
Simplify the contact detection in Ig2_GridConnection_GridConnection_GridCoGridCoGeom.
Clean up in header inclusions.