Merge branch 'no_marktasks' into 'master'

doc/RTD/source/Task/adding_your_own.rst

doc/RTD/source/Task/adding_your_own_neighbour_loop.rst

src/Makefile.am

src/cell.c

src/cell.h

src/engine.c

src/engine.h

src/engine_marktasks.c

src/scheduler.c

src/scheduler.h

src/task.h