Arrays in strings

[phyy-nx]

New fairmat applications and #1577 have attributes where the value is specified as an array expressed as a string. See e.g.:

https://github.com/nexusformat/definitions/blob/main/applications/NXxps.nxdl.xml#L86C22-L86C46

This convention is not expressed in our specification, so we need to define it some where so validators can check for it.

[mkuehbach]

• "[0, 1, 1]" NX_NUMBER, how to unpack?
• "['.', 'radial_axis']" NX_CHAR, how to unpack? Array of strings degenerated and forced to a string given XML lists are expressed as string, while in YAML list is possible
• "['.', 1]" NX_CHAR_OR_NUMBER, how to unpack?