choose_file GUI is used to select and open input files.
It contains the following three lines:
The choose_file GUI is defined by the file choose_file.tcl.
Filename button to use the
file selection dialog GUI.
Directory button to use the
directory (folder) selection dialog GUI.
Open button to open the file.
This can also be done by pressing the Enter (a.k.a Return)
key in the Filename entry field.