VoltageLevelFilter's getVoltageLevels function is now public (#620)

Signed-off-by: BOUTIER Charly <charly.boutier@rte-france.com>

network-area-diagram/src/main/java/com/powsybl/nad/build/iidm/VoltageLevelFilter.java

@@ -37,7 +37,7 @@ public int getNbVoltageLevels() {
         return voltageLevels.size();
     }
 
-    private Set<VoltageLevel> getVoltageLevels() {
+    public Set<VoltageLevel> getVoltageLevels() {
         return voltageLevels;
     }