Auto-build environment when turning on python mode

If it doesn't already exist

Author: hinerm

src/main/java/org/scijava/plugins/scripting/python/OptionsPython.java

@@ -187,10 +187,13 @@ public void save() {
 			log.debug(exc);
 		}
 
+		if (pythonMode && (pythonDir == null || !pythonDir.exists())) {
+			rebuildEnv();
+		}
 		// Warn the user if pythonMode was just enabled and wasn't before
 		if (!initialPythonMode && pythonMode && uiService != null) {
 			String msg =
-				"You have just enabled Python mode. Please restart for these changes to take effect!\n\n" +
+				"You have just enabled Python mode. Please restart for these changes to take effect! (after your python environment initializes, if needed)\n\n" +
 					"If Fiji fails to start, try deleting your configuration file and restarting.\n\nConfiguration file: " +
 					configFile;
 			uiService.showDialog(msg, "Python Mode Enabled",