Iris snippets

@rsignell-usgs  this notebook has some code snippets and helper functions to deal iris.  I would like your opinion to improve them.  (Also to test  with a greater variety of models.)

What it does:
- Get **just one** cube from a URL using a compatible *standard_name* list and constraint it in *time* and *space* standardizing the units (`get_cube(url, name_list=None, bbox=None, time=None, units=None)`).
- Find, hopefully the canonical one, *time* and *vertical* coordinates (`time_coord(cube)` and `z_coord(cube)`).
- Performs 1D and 2D *horizontal coordinates* extraction (`intersection(cube, bbox)`).
- Snippet to find and plot the surface level.

What needs to be done (add what you think is needed here):
- Deal with non geographical coordinates.
- Add an option to use *standard_name* and/or *long name* when creating the name_list.
- Slice using *datetime* objects.
- Add the *KDTree* search.