Compute the correct particle ID in the gravity force checks also for the SPH particles and star particles.

Various issues, now reproduces the plots we produced before, also have a new mpiactive column

Only check the matter content against the cosmology if both cosmology and self gravity are activated.

Added GitHub README

See merge request !517

Add a function to check at start-up that the mass content of the simulation matches the given cosmology.

Added ability to select the number of particles in the initial conditions as a runtime parameter for Evrard Collapse

See merge request !518

More cosmology work

Closes #407 and #404

See merge request !516

# Conflicts:
#   src/scheduler.c

Copy the correct arrays when generating gas from ICs. Also set the correct counters in space_init().