`ggap.nc` examples fail w/ downloaded data due to indexing

Since providing download access to the sample datasets used in the examples in #113, and setting the test suite to use those exactly for consistency, I noticed lots of new test failures. Upon investigation, it is only examples that use the ggap.nc dataset which are newly failing, with the cause being that the wrong field is being picked out from the read-in fieldlist - the index used is taking the wrong field now, for some reason (somehow in my local sample data, the fields got in a different order in the list I imagine).

This highlights the brittleness mentioned in #119, and by addressing that Issue, it will fix the failures and make the examples and tests more robust to boot.

Image comparison diff evidence

As examples of corresponding image comparison diff failures, showcasing that each output is generated and seems consistent, but shows the wrong (part of, or set of) data, see below.

Example 19b

Example 19c

Start with the examples that use the downloaded ggap.nc dataset and the image-comparison failures for Examples 19b and 19c. Read issue #119 for the related brittleness context, then run the affected examples and tests; done means they select the intended data fields and pass consistently with the downloaded samples.