ENH: Fixed false warnings during CLI execution

This warning was logged many times when a CLI module that had markups input/output was executed:

Warning: In C:\D\N\Slicer-1\Modules\Loadable\Markups\MRML\vtkMRMLMarkupsNode.cxx, line 1142
vtkMRMLMarkupsFiducialNode (000001682BBBE400): SetNthMarkupLabelFromStorage: no storage node to do the conversion!

It was because the GetNthMarkupForStorage and SetNthMarkupFromStorage methods called the storage node, which
does not always exist.

As only the storage node calls GetNthMarkupForStorage and SetNthMarkupFromStorage and
the necessary ConvertStringToStorageFormat and ConvertStringFromStorageFormat methods are already in storage node,
it is simpler to just call these converter methods directly in the storage node.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25327 3bd1e089-480b-0410-8dfb-8563597acbee