Docs (tutorial): add context for referenced functions

[sadielbartholomew]

(As per #17, I will add a commit to build changes into the HTML files, once I can work out why I am getting errors trying to build the documentation.)

In the 'Inspection' section of the documentation tutorial there is reference to the built-ins repr() & str(), but in the subsequent code examples those built-ins are not called directly, rather called under the hood by calling, & by calling print() on, respectively, the field construct (or dataset, etc.).

To someone without relevant knowledge of Python to know that print() show the str() etc. I think it could be unclear as to how the code snippets & the functions referred to relate, so I suggest adding something similar to that proposed here to put that into context.

[davidhassell]

Thanks for these changes - they do make things clearer.

It's probably best to build the docs (when you fix the path problem ...) with something like make html dev, as any changes to the html created by running make html . will, if committed, appear in the latest docs at https://ncas-cms.github.io/cf-python/, which would then include documentation for features that are not in the latest release (3.0.5).