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.