Fof io and compile without fof

A bunch of FOF-related changes:

• Put the gpart-carried fof-related variables into a structure.
• Move the FOF-i/o functions to a separate file
• Write the FOF groupID for the spart and bpart as well as for all the hydro models
• Write the groupID as a long long and not an int
• By default compile the code without FOF
• Add a configuration option to compile with FOF (enabled when EAGLE is switched on)
• Simplify the io_properties construction code

This allows to save memory on DM runs where we don't want FOF and hence don't need to carry the GroupID. Also makes the i/o work in all cases.

added 30 commits

• b292ca2d...027223b6 - 24 commits from branch master
• 81fb6fe9 - Simplify the construction of i/o properties and add the option to have an i/o…
• 4617029a - Moved the particle-carried FOF fields to a separate structure. Write the groupID…
• 5e482a41 - Use the correct number of fields in the Default gravity i/o call.
• f8f1f502 - Compile the FOF-related functions only if WITH_FOF is defined.
• f79e1af1 - Renamed the HAVEFOF configure constant to HAVESTANDALONEFOF
• 104e3f71 - Added option to configure the code with or without FOF activated.

Compare with previous version

assigned to @pdraper

This should just be a matter of code health.

merged

mentioned in commit 0c47ff92