Update specs to 1.8

[eboileau]

A clear and concise description of todo items.

• [x] Update related docs, incl. PDF link.
• [x] Update specifications.py::SPECS_EUF (some key:value pairs might be unused now e.g. constraints or columns)

[eboileau]

How does that affect existing datasets, e.g. on download?

[eboileau]

My initial intention was to "decouple" the specs from the source code (at least for changes that do not affect the database), but this was lost after a number of commits...

Now, data import implicitly assumes that specs are backward and forward compatible, e.g we can read both 1.7 and 1.8 and any future version. This is relevant for the header definition only, as the rest is handled by the data models and DTOs. Data is exported using the latest version, also assuming backward and forward compatibility, i.e. dataset in the database can be any version, but are always written using the latest one.

If this is always the case, we could simplify the SPECS_EUF definition. But if we eventually have some incompatible changes (minor changes not affecting the database), then these changes are now tied to the source code, and we must make a major release upgrade.