Address API documentation summary line TODOs

All methods should at least have a summary line to outline their basic purpose or usage. Currently this is not the case leaving the rationale for methods such as Field.get_coordinate_reference (as noted in #239) potentially unclear.

As a crude estimate of the number of methods undocumented minimally, git grep '"""TODO' | wc -l under the cf/ dir gives 99 for the state of the codebase at time of making this comment (with the equivalent for "'''TODO" not required as black enforces double-quote marks for docstrings etc.). So ~100 methods need summarising. That's small enough to tackle for the next release, easily.

Inizia nella directory cf/ eseguendo git grep '\"\"\"TODO' | wc -l per individuare i metodi privi di docstring riepilogative. Leggi ogni metodo interessato, incluso Field.get_coordinate_reference, per comprenderne lo scopo di base e l'utilizzo. Il lavoro è completato quando le docstring TODO contengono righe di riepilogo concise e il conteggio rimanente è stato gestito.