Append mode: address domain ancillary limitation

[sadielbartholomew]

Decide on, and implement, a solution to address the issue outlined in the items labelled (1) regarding domain ancillaries and the corresponding formula terms in #69 (comment).

A successful solution can be confirmed by including an iteration concerning appending the n=1 example field (the only example field with domain ancillaries) to the test_write_netcdf_mode unit test and it passing, rather than failing due to an extra field being present and/or the field in question as appended not possessing the right domain ancils. For that reason, the append mode of #69 excludes that example field from the unit test.