Add show_progress() to globally suppress MGT progress bars

[Copilot]

Progress bars are useful interactively but break batch pipelines and server logs due to repeated \r writes. This adds a global toggle to suppress them.

Changes

• musicalgestures/_utils.py: Adds _SHOW_PROGRESS module-level flag (default True) and show_progress(enabled: bool) function. MgProgressbar.progress() short-circuits silently when the flag is False, while still advancing finished state correctly.
• musicalgestures/__init__.py: Exports show_progress at the top-level package.
• tests/test_utils.py: Adds Test_show_progress (8 tests) covering flag state, output suppression, finished tracking, and round-trip enable/disable behaviour.

Usage

import musicalgestures as mg

mg.show_progress(False)  # suppress all progress bars

for video in my_large_dataset:
    result = mg.MgVideo(video, starttime=5, endtime=15)  # silent

mg.show_progress(True)   # re-enable for interactive use
mg.MgVideo(single_video)  # progress bar visible again