Snapshot offsets

Adds a "hash-table" to the snapshots containing the information about the location of particles in the snapshots based on the content of the top-level cells used internally by the code.

I have now implemented the mechanism also over MPI and documented it.

@jborrow let me know what you think of the description. I'll document the rest of the snapshot stuff when I have nothing better to do...

Great, thanks!

assigned to @matthieu

added 2 commits

• b32f33ab - Added units to the gravity and hydro parameter outputs in the snapshots.
• 4b3072b4 - When writing the boxsize and time to the snapshot headers, convert the values to…

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added 1 commit

• 90c83959 - Do not assume the box is cubic when writing the box size to the snapshots.

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merged

mentioned in commit b7989017