Increase the emphasis on writing and running function

[raacampbell]

After this session, some students remained unsure about the process of creating a function in the editor, giving it inputs and outputs, then calling this at the command line. Even if we don't get to if statements, the way functions work is critical and needs to be clearly conveyed.

[raacampbell]

Again, in 2017 this was an issue when we got to the Ca++ imaging. Need more time on this.