allow string array content

[prjemian]

need to allow string array content in addition to standard strings (an enhancement to NeXus)

in some instances, strings may be limited to rank=1 (which is, in effect, what exists now)
in some cases, we can be less restrictive, up to NX_RANK_MAX

[prjemian]

What work is required to close this ticket? Perhaps this added to the manual in the right place:

Clients should support string arrays. When shape is (1,) then that means there is only one string and the string array can be flattened to just string.

[zjttoefs]

The @axes does make use of it, doesn't it? Then there should be no work required.

[prjemian]

The datarules.rst file is one place to add the text under the strings definition.

http://download.nexusformat.org/doc/html/datarules.html#nexus-data-types

Also, to be like the other definitions nearby, start this definition with

NX_CHAR:

Could also expand the documentation of NX_CHAR in nxdlTypes.xsd