Added a maximal smoothing length that can be used to prevent particles from becoming too big in vacuum cases near the box edges. Will break the accuracy of the hydrodynamic scheme but is necessary in some situations.

Compute the total mass in the system and add a variable to the gpart to record how much mass they interacted with via self-gravity.

Cleaned up Gizmo fixes. Added a hydro_timestep_extra method and added an empty version for all hydro schemes.

one big messy commit for the status quo, forces sorts of the active cells in each step, does not use task_type_sub_*, and probably sorts some cells twice. but it works.

Revert "Force a sort of the particles at every time-step for active cells and at reception of data."

This reverts commit 078d450f