Only initialize gateway server when the tool is actually used.

Otherwise, the server is started for any itools command, and never shutdown,
causing the command to block at the end of the execution.

py-powsybl/src/main/java/com/powsybl/powsybl/PyPowsybl.java

@@ -20,8 +20,6 @@
 @AutoService(Tool.class)
 public class PyPowsybl implements Tool {
 
-    final GatewayServer server = new GatewayServer();
-
     @Override
     public Command getCommand() {
         return new Command() {
@@ -58,6 +56,8 @@ public String getUsageFooter() {
     @Override
     public void run(CommandLine line, ToolRunningContext context) throws Exception {
 
+        GatewayServer server = new GatewayServer();
+
         Runtime.getRuntime().addShutdownHook(new Thread() {
             public void run() {
                 System.out.println();