move synthseg submodule to code folder

for compliance with yoda principles

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "synthseg"]
-	path = synthseg
+	path = code/synthseg
 	url = https://github.com/HippocampusGirl/SynthSeg.git

code/build-synthseg-container.sh

@@ -8,7 +8,7 @@ base_directory=$(git rev-parse --show-toplevel)
 docker buildx build \
     --tag="synthseg:latest" \
     --load \
-    "${base_directory}/synthseg"
+    "${base_directory}/code/synthseg"
 singularity \
     build \
     --disable-cache \