List container improvements and testing

- support range based for loops
 - add remove() method to remove a node
 - add clear() to empty entire list and delete nodes
 - add empty() helper

src/systemcmds/tests/CMakeLists.txt

@@ -48,6 +48,7 @@ set(srcs
 	test_int.cpp
 	test_jig_voltages.c
 	test_led.c
+	test_List.cpp
 	test_mathlib.cpp
 	test_matrix.cpp
 	test_microbench_hrt.cpp