In engine_marktask() only check whether to rebuild the space for density pair/sub-pair, not for all sub-types of pair/sub-pair.

Only free tp->threads when allocated, set the mapper function for logging when only using one thread

Make sure that a repartition type of none does nothing, including triggering side effects like drift-all each step

and they are not for one thread

and they are not for one thread

This reverts merge request !375

Partially restored old time step criterion. Isolated flux limiter into a separate file and made it configurable. Removed some superfluous gravity variables.

Moved around the energy gravity update so that it uses the old momentum. Removed a superfluous time step in mflux.

remove tests for undrifted particles in send/recv as not all sent/recvd particles are drifted, only those that are needed. note that undrifted particles will still be caught in the interaction functions.

keep a separate log buffer per thread id. this avoids having to protect against concurrent access to the logs.