When appending, make the Tecplot Outputter overwrite the file the
first time it writes.

This prevents multiple runs in the same directory from appending to
an old data file and giving misleading results.

Refs #440.
Merge pull request #2820 from jwpeterson/tecplot

When appending, make the Tecplot Outputter overwrite file first time
Merge commit '1bda85eab14d2806ac491b025d6ecbfece0b7368'