fix windows compilation warning
hide stl in the implementation instead of exporting it in the header to adress visual studio warning C4251 'needs to have dll-interface to be used by clients of the class'.
Backport: paraview/release
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hide stl in the implementation instead of exporting it in the header to adress visual studio warning C4251 'needs to have dll-interface to be used by clients of the class'.
Backport: paraview/release