Can now run through all the tasks but still missing some interactions coming from the close-by pairs over MPI

All-reduce all the top-level multipoles after a rebuild() to make sure we have all information in hands to construct the tasks.

Also send dx_max_part and dx_max_gpart alongside ti_end_min at the end of a time-step when running with MPI.

Added debugging checks to verify that the particles are in the correct frame after the shift in DOPAIR1 and DOPAIR2. The check takes into account directions and drift since last rebuild.

Fixed the gravity bugs reported in #363. Need to find a good rebuild criterion for gravity-only runs.

Fix to #363. Drifting decision needs to use the same multipole information as the actual tree walk. Also fix some gravity_cache padding values.

Added the missing DOSELF1_NAIVE() interaction function. Added checks that particles are drifted also in the subset functions.

their Initial Conditions files. This avoids error rasises from H5G* when
opening files without units and reverting back to the ones specified in
the parameterfile. We now also let users know when their units are being
read from their ICs explicitly.

Applied @jwillis' correction to the second loop of DOPAIR2. dx_max was entering the external loop with the wrong sign.

Modify the inner-loop exit condition to incorporate the distance on the axis calculation rather than pre-compute it.

There is a problem when the cpu mask is empty and using multiple MPI ranks on a node, in that case all the main threads end up pinned to one core during the reading of the initial condition (and any other tasks done before engine_init is called).

In DOPAIR2(), once we have a pj candidate, check the distance on the axis using hj instead of relying only on the earlier hj_max check.

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.

Reinstate missing pointer indirection for external gravity dependencies that had disappeared in merge.