Add a simple mp4 parser to replace pymp4

[ptpt]

Why

• pymp4 uses an old construct version Update to construct 2.9 beardypig/pymp4#3
• pymp4 can't handle mp4 files larger than 4GB: Large size not handled beardypig/pymp4#15

Tests

This PR also adds test that cover the parsing (test data copied from pymp4):

python3 -m pytest -s -vv tests/unit/test_simple_mp4_parser.py

Also test against real mp4s:

# should not see any diffs
diff <(python3 -m mapillary_tools.geotag.blackvue_utils_pymp4 ~/BlackVueVideos) <(python3 -m mapillary_tools.geotag.blackvue_utils ~/BlackVueVideos)

Fixes #439