phonons.py: BUG in get_band_structure method and MAINT

Checklist

• I have read the contribution guidelines.
• Unit tests have been added for any new or changed code, and the test suite passes. Note: Your request will likely not be merged without the appropriate tests.
• "closes #XXXX" is in the body of the MR description to link the related issue, if applicable.

Description

BUG: Improper assert.

MAINT: Add proper return control.

There was a faulty assert 0 statement in control check for modes variable. Additionally, if modes was set to True no value was returned. These have been fixed and added short description of the method.

New unit test in ase/test/test_phonons.py for get_band_structure method of Phonons. All 5 test passed.