Misleading warning for reference

[EmmaRenauld]

We now have this thing in load_tractogram_with_reference if a .trk is loaded with a reference.

That's important is some case and very misleading in others.

Ex: Do pytest --no-cov -rP src/scilpy/cli/tests/test_bundle_filter_by_occurrence.py. The output is:

But no reference was passed. But default, the script uses a reference to loop on files. This is annoying in scripts that loop on many files and use the reference even if the file is a .trk.

[EmmaRenauld]

@arnaudbore Not sure how to fix this