Python: Move bundled netCDF4 submodule

Very importantly, this avoids the `examples` directory, with its 134MB
of `.nc` files, getting copied into the wheel

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "src/nc_complex/netCDF4"]
-	path = src/nc_complex/netCDF4
+	path = external/netCDF4
 	url = https://github.com/PlasmaFAIR/netcdf4-python.git

src/nc_complex/CMakeLists.txt

@@ -27,31 +27,34 @@ set_target_properties(_c_nc_complex PROPERTIES
   BUILD_WITH_INSTALL_RPATH TRUE
   )
 
+# Path to the netCDF4 submodule
+set(_external_netCDF4_dir "${CMAKE_CURRENT_SOURCE_DIR}/../../external/netCDF4")
+
 # We make a copy of the bundled _netCDF4.pyx in order to give it a
 # different name and avoid clashes with the original when importing
 # (even though this is in a different namespace, because it's a
 # compiled module, it'll end up with the same name for the actual .so,
 # and we have fun with linking/dynamic loading)
-configure_file("${CMAKE_CURRENT_SOURCE_DIR}/netCDF4/src/netCDF4/_netCDF4.pyx"
+configure_file("${_external_netCDF4_dir}/src/netCDF4/_netCDF4.pyx"
   "${CMAKE_CURRENT_BINARY_DIR}/_pf_netCDF4.pyx"
   COPYONLY
 )
 add_custom_command(
   OUTPUT _pf_netCDF4.c
-  DEPENDS netCDF4/src/netCDF4/_netCDF4.pyx
+  DEPENDS "${_external_netCDF4_dir}/src/netCDF4/_netCDF4.pyx"
   VERBATIM
   COMMAND "${CYTHON}" "${CMAKE_CURRENT_BINARY_DIR}/_pf_netCDF4.pyx"
           --output-file "${CMAKE_CURRENT_BINARY_DIR}/_pf_netCDF4.c"
           --include-dir "${CMAKE_CURRENT_BINARY_DIR}"
           --include-dir "${CMAKE_CURRENT_SOURCE_DIR}"
-          --include-dir "${CMAKE_CURRENT_SOURCE_DIR}/netCDF4/include"
+          --include-dir "${_external_netCDF4_dir}/include"
           --3str
 )
 
 python_add_library(_pf_netCDF4 MODULE
   "${CMAKE_CURRENT_BINARY_DIR}/_pf_netCDF4.c"
   WITH_SOABI)
-target_include_directories(_pf_netCDF4 PRIVATE "${CMAKE_CURRENT_SOURCE_DIR}/netCDF4/include")
+target_include_directories(_pf_netCDF4 PRIVATE "${_external_netCDF4_dir}/include")
 target_link_libraries(_pf_netCDF4 PUBLIC netCDF::netcdf PRIVATE Python::NumPy nc_complex)
 set_target_properties(_pf_netCDF4 PROPERTIES
   INSTALL_RPATH "$ORIGIN"