Native item order option in get_itemnames()

[stefvanbuuren]

The get_itemnames() function returns the item names of one or more instruments. Most users want or expect the sequence of items to match the published instrument.

Although the order argument of get_itemnames() can change the item order, the current implementation is not flexible enough to reproduce the published sequence. It would be useful to have an option "native" in get_itemnames(), set as the default, that returns the items in the order in which they were published.

Possible complications:

• A native option can only work if different versions (e.g., translations) of an instrument use the same items in the same order. This is a strong assumption and may not hold for many instruments.
• We would need to encode the published order for every supported instrument, which requires looking up the item sequence for each and implementing a mechanism that makes this transparent to users.