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/