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
ENH: Improve code coverage for UnicodeStringArray

Added tests to TestUnicodeStringArrayAPI, bringing coverage from 24%
to 100%.

Change-Id: I48b36bc47090a2966262e2dfd1bb5375024ca078
Re-enabling tests disabled during modularization for Interaction/Widgets

Fixed dependencies and added baseline

Change-Id: I6621bd523e8811568416a2c1d4e9279b8a6a4f27
Merge topic 'COVERAGE_InteractionWidgets' into master

942444bf Re-enabling tests disabled during modularization for Interaction/Widgets
Merge topic 'Coverage_UnicodeStringArray' into master

94a63119 ENH: Improve code coverage for UnicodeStringArray
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
fix the format of PhyloXML references

The PhyloXML specification indicates that references should
match the following regular expression pattern:

[a-zA-Z0-9_]+:[a-zA-Z0-9_\.\-\s]+

In order to do so, we now prepend "VTK:" to the beginning of the
array name.

Change-Id: Ie898e19dc10ec6edd2b67a5b493f55521e15c63d
Merge topic 'phyloxml_reference_format' into master

183bb5d7 fix the format of PhyloXML references