follow approach of scitools/iris course for examples

In the scitools courses e.g. https://github.com/SciTools/courses/blob/master/course_content/iris_course/1.The_Iris_Cube.ipynb

the approach is to put each example in it's own file, and then include a way to load it inside the relevant worksheet using:

SAMPLE SOLUTION:
Un-comment and execute the following, to view a possible solution, and some code.
Then run it ...

```
# SAMPLE SOLUTION
# %load solutions/iris_exercise_1.2a
```

this has two advantages:
1. it makes it easier for the user to access the relevant answers
2. it make it easier to test (no need to cut and paste the solutions in)

Start by comparing the referenced course_content/iris_course/1.The_Iris_Cube.ipynb with the relevant PyPRECIS worksheets and examples. Use the solutions/iris_exercise_1.2a loading pattern as the model, and consider the work complete when examples are separately accessible, loadable from the worksheets, and runnable without manual copy-and-paste.

Written by the indexing model from the issue text.