feat: add ludt box support

[tanghaowillow]

Add ludt, alou and tlou support.
Fixes #240

The only test file I could find was from MPEGGroup/isobmff#38.

[tobbee]

@tanghaowillow Thanks for the PR. I'm a bit tight on time right now, but I will try to go through it in the next few days.

[tobbee]

Great work!

The main issue I see is that non-zero loudness_info_type values are not detected and would lead to broken parsing. Returning an error for non-zero values may be a good enough for now, but just parsing the extra byte and storing the two values would be a cleaner solution.

The test file you found is good to have, but is not used in any test as far as I can see. Maybe just extracting the ludt box data and decode it could be a good test.

Please check my other comments as well.

In any case, really nice to receive such a good PR.

[tanghaowillow]

@tobbee Thanks for your advice. I have modified the code according to the comments.

[tobbee]

Looks good to me with the changes including the box test and the error for a non-supported value.

Thanks a lot for your contribution!

[tobbee]

@tanghaowillow
Found a minor thing in the automatic tests:
golangci-lint complains about an error value from AddChild not being used in the ludt.go file. I suggest that you remove the error value since it is never triggered.

[tobbee]

@tanghaowillow Your PR has now been merged. Good work!

Welcome back if you want to contribute with other code or file an issue!