Add the PTEP IMO to the source code distribution

[ziotom78]

This PR includes a “default” IMO file, containing the information that were published in the PTEP paper.

• [X] Add the JSON PTEP file
• [X] Add the JSON PTEP file to the list of redistributable files
• [X] Add the ability to read many (gzipped?) IMO files and merge them
• [X] Include the PTEP IMO in the distribution
• [x] Add tests
• [x] Update the CHANGELOG
• [x] Update the documentation

[ziotom78]

Finally, after nearly 7 months of work, this PR is done! I have updated some tests and the documentation, so that in many places we use the PTEP IMO instead of hand-made DetectorInfo/ScanningStrategy/etc. classes. The documentation should be more readable and the tests easier to parse.

The PTEP tree structure conforms to the so-called “IMO v2.0”, which is however not the IMO v2.0 that was discussed at Elba but a previous version (it should have been called IMO v1.4), whose structure is similar to IMO v1.3.

Please comment, I would like to merge this PR in a few days.