Endianness documentation and support

I don't see any mention of endianness in the docs or `fpzip -h`, and I'm trying to understand if it supports either endianness and if so how to toggle it. I tried it on both little-endian and big-endian versions of my data and got a much better compression ratio for big-endian data, so perhaps it only supports big-endian? Curious to understand better.

Start by reviewing the existing documentation and the `fpzip -h` output, then inspect the endianness behavior described in the issue on both little-endian and big-endian data. Document whether each endianness is supported, whether a toggle exists, and why the observed compression ratios differ, updating the relevant documentation or help text once confirmed.

Written by the indexing model from the issue text.