Removed the softened isothermal potential. The isothermal potential is now softened by default. Set epsilon to 0 to restore the old behaviour.

Drift on demand

Fix to the time integration bug that triggers #248. 

As written in the minutes, finding all the cells that need to be drifted can be really difficult in some convoluted situations and I'd like to avoid a full tree walk. 

My solution is as follows:
 - (Re-)Create a drift task. When a cell has active particles, it's drift task is active
 - The `runner_do_drift` function gets replaced by a `runner_do_unskip` function that does not touch the particles but just updates the status of the tasks.#
 - There is a `runner_do_drift_all` function that drifts everything for the cases where we need this (rebuild or snapshot).
 - If a pair task involves a cell that is inactive and has hence not been drifted, the pair task starts by calling the drift operation on this cell and then carries on with its regular work. Most of the drifts are done via the regular task though.

In terms of performance it looks fairly similar to the current master. But now it is physically correct.

See merge request !292

This reverts commit de558d53.

The non-MPI SodShock test starting using 0 dt steps.

Streaming io

Some infrastructure needed for particle logging, should not break anything as it's not used yet.

Next step is to write the bits that log particle data at different resolutions.

See merge request !297

Conflicts:
	src/Makefile.am

Fix doxygen error.

Completed documentation for `test_interaction` function.

See merge request !296

Add support for jemalloc

Since it is available.

See merge request !295

if the threadpool is initialized with a single thread, don't bother spawning a r…

…unner, just run in the threadpool_map callee, which is blocked.

See #252.

See merge request !293

Space recycle

Do the recycling not cell-by-cell, but with lists of cells at a time.

See merge request !291

if the threadpool is initialized with a single thread, don't bother spawning a runner, just run in the threadpool_map callee, which is blocked.