Creates the named log file and writes some header information to it, including the included units, runtime parameters, physical constants. Meta data about the run is also stored like a time stamp, the run dimensionality, compiler flags and more. The logfile can be stamped from any unit to store the simulation's progress. The name of the parameter file is taken as an input; it is echoed to the log file. Only the master processor actually writes anything. In order to avoid accidentally overwriting an important logfile during a science run, the logfile is always opened in append mode.