Added example of using GSSI 1.5GHz antenna in free space.

user_models/antenna_like_GSSI_1500_fs.in

@@ -0,0 +1,9 @@
+#title: GSSI 1.5GHz 'like' antenna in free-space
+#domain: 0.250 0.188 0.220
+#dx_dy_dz: 0.001 0.001 0.001
+#time_window: 6e-9
+
+#python:
+from user_libs.antennas import antenna_like_GSSI_1500
+antenna_like_GSSI_1500(0.125, 0.094, 0.100, resolution=0.001)
+#end_python: