Nifty clutser solution

See merge request !915

Fix an undefined behaviour problem

Closes #646

See merge request !1000

Fix an undefined behaviour problem in engine_split_particles() by using the more expensive (but correct) functions to relink the gpart to other particles.

Backport a fix from the COLIBRE repo to solve a segfault in the cleaning routines when ending a run that restarted from check-point files.

Time-step limiter and time-step synchronization over MPI

See merge request !999

Avoid overflow problems with the number of waits/unlocks of tasks. Now that we go beyong 64bits for tasks it makes no sense to worry about memory footprint for these anymore.

Logger: small fix

See merge request !998

Add -march=core-avx2 for AMD CPUs compiled using ICC

See merge request !993

From http://www.prace-ri.eu/IMG/pdf/Best-Practice-Guide-AMD.pdf, note we add these to CFLAGS as the icc_flags is a choice vector

Logger: add doc

See merge request !971

Sphenix SPH

See merge request !994

Speeds up self-gravity x2

Random: add seed parameter

See merge request !988

Io bug?

See merge request !992

Particle splitting "a la EAGLE"

See merge request !984

Add option to write each VR output to a separate directory

See merge request !934

This reverts commit 28265917, reversing
changes made to 856bcb95.

Add metallicity & sfr unit to vr configs

See merge request !989