Improve explanation of how to run code, and refrain from using run all

[jamescavanagh]

Week 0:

Students would often use run all because of the convenience.

This revealed a lack of understanding of how algorithms proceed and traditional coding.

It would be good for this distinction to be cleared up, even using an error message in the orignal notebook at the bottom to show them not to use it

@knectt Thank you for bringing this to our attention