add option to write XML to string instead of file

This commit enables vtkXMLWriter and its subclasses to write
their output to a string instead of a file.

Change-Id: Idff93039f9536379e3ef9abad6ea4ce18b4f2a60
fix failing tests revealed by gerrit

subclasses of vtkXMLWriter should now call OpenStream & CloseStream
as opposed to OpenFile & CloseFile.

Change-Id: I39ccdc711d07e96763fc941b5fcd80b1da6b38c5
Merge topic 'write_xml_to_string' into master

e90c46ae fix failing tests revealed by gerrit
52d62550 add option to write XML to string instead of file