Adding a convenience function to perform a
boost::dynamic_pointer_cast<> to AbstractFilter
Fix error in generated XDMF xml file

The xml comment was not properly terminated. This has been fixed.
Add Unit Test for VtkStructuredPointsReader

A unit test was added for the VtkStructuredPointsReader. This tests
basic scalars for both POINT and CELL data. We need to
add  writing/reading test for vector data in addition to adding code to
actually read VECTOR data (numcomps = 3) from the vtk file.