VecCore

This is the new merge request for VecCore. Please provide your feedback here.

The veccore branch will now be regularly rebased onto master and this merge request will be updated accordingly.
Features not yet ready for inclusion into VecCore will be kept in separate branches, all based on the veccore or veccore-devel branches.

Note:
In order to avoid polluting the VecGeom repository with my personal branches and with tons of emails when I rebase and push, I created a fork of the repository in CERN's GitLab. Let's try this forking model for personal branches and see if it works better than having all branches in the same place.