Complete greeting and circle_area scripts.

- A value can have many different types but a name is just a reference to a value.
- When you are filling out a form you are providing values for names. If the form
asks you to do something like "combine lines 1 and 3" your answer better not be "line 1 line 3".
- When you need to repeat a value, like the area of a circle, many times it is easier
and more efficient to save the value to a variable. It also makes code more understandable.

greetings.py

@@ -2,6 +2,6 @@
 # ------------
 # By MWC contributors
 
-my_name ="Chris"
-greeting = "Hello, " + my_name
+user_name = input("What's your name? ")
+greeting = "Hello, " + user_name
 print(greeting)