sofar: Refactoring - Roadmap for sofar 1.4.0

Suggested behavior:

• Add Sofa.convantion_status property that returns 'current', 'preliminary' or 'deprecated'

Reading SOFA files and creating Sofa objects

• Don't raise warnings if reading preliminary or deprecated conventions anymore. Thats cluttering the command line and is only important when writing
• Verify preliminary conventions. They should become stable at some point and errors should come up to be fixed. They are currently not verified. I forgot the reason for this.

Writing SOFA files

• Print information about custom variables with the information that users should check if they are necessary of information should be moved to standardized variables.
• Make printouts optional via a verbose flag
• Improve warning message for preliminary convention. Currently it is mentioned that conventions should be updated if possible. Which is actually not possible.

Updates in conventions

• [ ] Update conventions, code, and testing once the conventions are released (see #112)

Start by reviewing the SOFA reading and writing paths, convention handling, and existing tests; check issue #112 for the pending convention release. Treat the checklist as a roadmap: done means the convention status, warning and verbose behavior, custom-variable messaging, and released-convention updates are implemented and covered by tests.