Added submodule for CTAB-GAN.

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "external/LoRAS"]
 	path = external/LoRAS
 	url = https://github.com/COSPOV/LoRAS.git
+[submodule "external/CTAB-GAN"]
+	path = external/CTAB-GAN
+	url = https://github.com/Team-TUD/CTAB-GAN.git