Added communication functions to propagate the new number of star particles after a SF event has occured.

Prevent users from running with cooling switched on and no minimal temperature in the parameter file.

Only initialise the potential after the space has been initialised as the potential requires the size of the space.

Added basic support for black hole particles. Particles can be read in, attached to gpart, integrated forward in time, drifted and written to snapshots.

Prevent users from running with cooling or star formation if no such model was specified at compile time.

Authorize the creation of new stars over MPI. Extra space is allocated at the end of the local cells only.

Move the memory allocation/deallocation functions into the header and inline and make appropriate other changes as suggested by Pedro

Formatting