Fix subsetting on Nids files.

ArrayStructure gained support for sections now, and use this support to
actually use the sections rather than returning the entire array. The
lack of this was breaking, at a minimum, the data table for these
variables in ToolsUI.
Fix reading text information from graphics block.

Copy paste bug. Removes stack traces when using ncdump/datatable on
these type of variables in ToolsUI.