Timestep order

This is my last batch of changes regarding time integration.

• Additional checks here and there that the particles are synchronized. Especially gpart.
• Re-arranged the time-step sequence and simplified the logic in engine_step().
• Moved the decision of repartitioning from the main() to engine_step().

The re-arrangement of the time-step sequence allows for perfect energy conservation even when we rebuild, repartition or dump snapshots. Previously that was not the case as a spurious drift had to be introduced.

Added 1 commit:

• f1210eaf - Only force a repartition if you have METIS activated.

Added 1 commit:

• 351fa6bf - Parenthesis mix-up

Added 13 commits:

• 351fa6bf...7bd06f71 - 12 commits from branch master
• 1aaf8f0d - Merge branch 'master' into timestep_order

Added 16 commits:

• 1aaf8f0d...039540fd - 15 commits from branch master
• a84b9c7f - Merge branch 'master' into timestep_order

Added 4 commits:

• a84b9c7f...c707ab6b - 2 commits from branch master
• cbd001ff - Merge branch 'master' into timestep_order
• f181430f - Code formatting

This is harmless and blocking some more gravity updates so I'll baldly accept it.

Status changed to merged

mentioned in commit a679332b