Fully implement the timestep limiter (non MPI)

Closes #196

See merge request !956

Change the default primoridal Helium fraction to the value from WMAP7 that was assumed in all the EAGLE and OWLS runs.

In the black hole reposition code, calculate a delta position rather than an absolute position. This correctly handles the periodic boundary conditions. Also update the x_diff of the BH particles.

Reset the reposition variables once the reposition is done. Avoids problems when there are multiple drifts.

Change the EAGLE SNIa DTD to a properly normalized power-law of slope -1

See merge request !954

Only check the calculation of the minimal neighbour time-bin if we are not using vectorization. This feature is not used in the Gadget-SPH SIMD code.

Revert "When dumping a restart file at the end of a run, only drift the particles if no other form of i/o has taken place this step to avoid a drift towards the past."

This reverts commit d93c56e9.

Do *NOT* use the default values as optional parameters when re-reading the space global variables when resuming a simulation!

When dumping a restart file at the end of a run, only drift the particles if no other form of i/o has taken place this step to avoid a drift towards the past.

Change task order v2

See merge request !955