Field selection: use select_by_* methods instead of indexing #119

The docs examples always select the relevant fields from the read-in Field List using indexing, but this is less explicit and more brittle than selection using one of cf-python's dedicated select_by_ methods, so we should replace any cases where the Field List is non-singular with filtering by selection method.

I suggest that we still use indexing for those singular Field Lists to highlight that indexing is an alternative possibility which represents the simplest way to extract the field when it's the only one there, but otherwise always use some select_by_* method.