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

Implements the solution to #367. Instead of just sending the time-step we send the distances moved by any part and gpart in that cell. 

I have also done some cleaning up around the packing and unpacking and improved the documentation.

See merge request !434

Correct vec neighbour find

Fixes neighbour find bug in the vectorised version of the density interactions.

See merge request !435

Added an extra test case to testActivePair.sh to include the RNG seed that creates a vectorised neighbour find bug.

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

Fixes to the gravity code

This fixes #363. The long-range gravity task has to consider all the cells that were assigned to it at the rebuild time. 

Additional improvements:
 - Drift the multipoles while activating the tasks,
 - Avoid FPEs in the FFT task,
 - Adjust the number of tasks per top-level required to run.

See merge request !430

Small cosmetic improvements to the documentation of the FMM code.



See merge request !432

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.

Corrected the same check in the vectorized versions of the pair interactions. This was not using the correct criterion for the maximal displacement any particle can have had since the last rebuild.

See merge request !431

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.

Updated tolerances for GCC compilation.



See merge request !429

Run naive interactions

Added an option to run the naive interactions instead of the optimized ones. These are to be used for debugging purposes.

See merge request !428

Patch vectorised pair interactions

Implements small optimisations to `runner_dopair1_density_vec` suggested in #354:
 - Break out of the loop checking for active particles if the cell itself is inactive
 - Do the calculation in the frame of `cj` and not `ci` since this is where the shift vector brings us.
 - Correctly apply the rshift value on the axis linking cells.

See merge request !407

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.

Use H5Lexists to check whether /Units exists in IC files before attempting to read them

Updated to use H5Lexists to check whether user has input units into 
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.

See merge request !427