Add xml formatting CI test

[cssherman]

What is the requested feature? In develop, there is now a make target that will format all of the files in the src and examples folder: make geosx_format_all_xml_files . Randy suggested incorporating this into a CI test.

Note: in order to build the formatting tools, the version of python used to configure the code needs to have the virtualenv package installed (which is pretty standard but not part of core). Alternately, we can build the tools off of a separate python distribution by setting a cmake variable (PYTHON_POST_EXECUTABLE).

[paveltomin]

this can be closed, right?

[paveltomin]

@cssherman ?

[paveltomin]

@TotoGaz can this be closed?

[cssherman]

@paveltomin - Unless I'm mistaken, we are validating the xml files against the schema, but are not checking xml formatting (indentation, etc.) as part of the CI