Tecplot: Re-add the default ascii format extension of dat.

The tecplot manual states that the default ascii file format extension
is .dat, which we are missing from our tecplot readers for some
unknown reason.

Change-Id: I4492576d31f403d6b8141e535eca157d3b803020
Update the VisItBridge to update tecplot reader extensions.

Change-Id: I4492576d31f403d6b8141e535eca157d3b803020
Merge topic 'tecplot_extensions' into next

3d4785b Update the VisItBridge to update tecplot reader extensions.
e797c98 Tecplot: Re-add the default ascii format extension of dat.