Add mmap_mode option to allow writing to mmaped PLY files

[nh2]

Hi, @chpatrick wrote this and we find it super useful for being able to write PLY files in-place with mmap.

We already use this for a year in production and want to upstream it.

I just rebased it onto your latest master.

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📚 Documentation preview 📚: https://python-plyfile--79.org.readthedocs.build/en/79/

[dranjan]

Thanks for the contribution!

What should happen if the user requests pass-through writing but the file can't be memory-mapped? I'm actually not sure. My gut instinct is we should fail explicitly, but that could be a problem if the user is only interested in one element and it's some other element that can't be memory-mapped.

To take this to the finish line,

• There should be at least one unit test that exercises the feature in a nontrivial way.
• This should probably also be mentioned on the documentation site where it talks about memory mapping: https://python-plyfile--79.org.readthedocs.build/en/79/usage.html#faster-reading-via-memory-mapping The source for that is under the doc/ directory, and there are some (admittedly terse) hints regarding that under "Information for Developers" on the documentation site.

I can take care of all that, but feel free to take a crack at it yourself. My time frame is approximately one week from now, but that could change due to external factors.

[dranjan]

This will be superseded by #81. (The commit will still be merged, just not through this pull request. 😸)