relax SVN archive

Modified StructureTextCtrl to not supply the directory arg to openfile().

The current working directory is used and changed if no directory arg is supplied to openfile().

Converted the openfile() args to keyword args.

Reverted the last revision (r10374) as the directory arg was lost.

The command used was:
svn merge -r10374:10373 .

This directory arg will be useful in other parts of the GUI.

Modified the operation of openfile().

The directory is now set to getcwd(), and any moving around in the file structure in the dialog by
the user causes the current working directory to change (using chdir()).

Modified the openfile() function to match the relax conventions.

The function comments were converted to the function docstring, comments were added to the code,
and the spacing was fixed.

Missing import.

Changed the open_file() method to only change variables in its own class.

Renamed the StructureTextCtrl method structure_pdb() to open_file().

All PDB file inputs now use StructureTextCtrl.