Object equality: doctest-able examples covering special cases

There is a lack of concrete examples in the API reference across the equals methods available on the various construct and data objects (see e.g. the end of the references here and here) so during #254 we agreed it would be good to add in a good range of examples, ideally eventually doctest-able (i.e. inclusive of set-up of the objects referred to), which in the process of verifying and documenting should enable us to ensure behaviour is known and consistent in relation to such aspects as:

• management of self-equality (a.equals(a)) and whether to short-circuit on copies (a.equals(a.copy()));
• handling of subtleties of data (including relevant keywords to toggle these), e.g: how to deal with masked arrays, NaN values, different data types with equivalent data, compression etc.;
• handling of potential subtleties of metadata (not explored so much as with the above in #254, but I don't doubt there are some).

Review the equals API references linked in the issue and the decisions from #254 first. Inventory the construct and data object equals methods, then define doctest-able examples covering self-equality, copies, masked arrays, NaN values, data types, compression, and metadata. Done means the documented examples can be run as doctests and the relevant behaviours are explicit and consistent.