ENH: Update OpenIGTLinkIF - Add new message converters. Fixes #3548 & #3738

Also update GUI to allow users to push MRML nodes (#3738)

From: Junichi Tokuda <tokuda@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23429 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Ensure DCMTK libraries are packaged.

This commit fixes a regression introduced in r23413 where call to
"find_package(DCMTK ..)" have been moved in the sub-project where
it was explicitly needed. By calling, "find_package(DCMTK ..)", it
now ensures that the variable "DCMTK_LIBRARIES" will be set.

Note that ideally DCMTK libraries could be installed leveraging the
fact the project is cmake'ified by installing the expected components.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23430 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Add test checking reference set to null or empty string are removed.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23431 3bd1e089-480b-0410-8dfb-8563597acbee
STYLE: Consistently use "referencedNodeID" parameter name

To make maintenance of the code easier, the same parameter name
is used across all MRML node reference functions.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23432 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: update to new LandmarkRegistration

Now with interactive ThinPlate registration!
- uses real-time nonlinear transform updates
- compatible with transform visualization

Other improvments and upgrades, like VTKv6 support and
generalization of the plugin architecture.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23433 3bd1e089-480b-0410-8dfb-8563597acbee