[wip] Add defining functions

Author: hermite2053

flow/defining_functions/default_arg.py

@@ -0,0 +1,6 @@
+def foo(x = 1):
+    print(x)
+
+n = 3
+foo(n)
+foo()

flow/defining_functions/none.py

@@ -0,0 +1,4 @@
+def foo():
+    """This function does not return any value."""
+
+print(foo())

flow/defining_functions/squares.py

@@ -0,0 +1,10 @@
+# the squars, n^2
+
+def square(x):
+    return x * x
+
+n = 15
+for i in range(n):
+    print(square(i), end = ' ')
+
+print()