Removed time step from flux computation. Removed (untested) flux limiter and unused variables in hydro_part. Discovered and fixed a possible bug in Gizmo particle velocities treatment in flux computation.

If the neighbour count goes above NUM_OF_NEIGHBOURS don't log the neighbour id but keep logging the neighbour count.

Corrected typo in Minimal-SPH implementation that prevents neighbour number convergence in some cases.

Implement more general conversion functions for i/o. Can now add one that does box-wrapping  of particles.

Use the equation of state functions in a few more places in the gizmo code to be more robust to changes.

Reverse the loop to find the exit condition in DOPAIR2() to go from the centre to the outside. Follows @jwillis' suggestion in #362

Force compiler to report shadowed variables. Removed all shadowed variables. Added the function scheduler_activate_send() to simplify the unskipping of MPI sends.