Add CI config

Description

This adds a basic CI setup to the project!

There were 4 changes:

• The addition of the CI config file.
• Some old tests and their data, predating the calibration team, were moved to a tests/legacy folder.
  It would be good to fix these eventually, but so many changes happened that they might not be relevant anymore...
• The existing tests were amended to work on CALLAB data, which are the only data xcaltst has access to
• Since we've added isort to the CI, I decided to fix the imports now, such that we start on a clean green base from now on.

This can be merged once the CI runner has been restarted and the tests reran, to ensure it's all green.

How Has This Been Tested?

Well, CI.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• We have updated the documentation accordingly here: https://git.xfel.eu/gitlab/detectors/pycalibration/wikis/Gitlab-CI-Runner

Reviewers

@ahmedk @hammerd @roscar