ServiceVariables: merge variables importation logic

[PaulFarault]

Which issue(s) this PR fixes

Fixes None

Simplifies the import of variables which was split accros multiple methods that were not reused. Current version feels a bit over-engineered, probably because at first we wanted to expose some public API with tdp-lib.

Agreements

• I hereby declare this contribution to be licensed under the Apache License Version 2.0, January 2004.
• I hereby agree to grant TOSIT a copyright license to use my contributions.

[PaulFarault]

We must be able to edit several var files at the same time (as open_var_files did).

[PaulFarault]

Done. I switched back to an open_files method that yield a dict of opened files.