Cleanup datalog output a bit.
* Add close/flush on exit in case we have pending data
* Use hash for comment characters to be compatible with gnuplot
* Report x/y/z/e position in array
Record simulation run-time data in file 'datalog.out' so it
can be analyzed after-the-fact or during the run.
This feature is evolving. Eventually it should be compatible with
some logic analyzer GUIs such as gtkwave or even gnuplot.
Phil Hord