Variables are referred to with Exercises in between

[stephenworsley]

For example: In the Iris course 3 (Subcube_Extraction), in section 3.2, the cubes variable is defined at the top of the section and is referred to at the bottom. Between this, there are two exercises where the user my redefine or change the cubes variable, the second of which involves loading from another file. When loading this other file, I made use of the same name cubes which meant that, later on, the notebook was referring to a different CubeList than it was expecting. It may be wise to redefine such variables when there is an exercise in between for better consistency.