[devel] Add licence text

Adds the copyright statements using:

pip install LbDevTools
lb-add-copyright --license-fn LICENSE $(lb-check-copyright --porcelain)

And a CI job to ensure the statement is present in all new files.

If using the above steps, currently my fork is required to pick up the changes in lhcb-core/LbDevTools!63 (merged):

pip install git+https://gitlab.cern.ch/cburr/LbDevTools.git@configurable-copyright