attrDataItem-methods: Get and set attributes values for items in an attrData object
In graph: graph: A package to handle graph data structures

The attrDataItem method provides get/set access to items stored in a attrData-class object.

1 2	attrDataItem(self, x, attr) attrDataItem(self, x, attr) <- value

`self`	A `attrData-class` instance
`x`	A `character` vector of item names
`attr`	A `character` vector of length 1 giving the attribute name to get/set. Note that the attribute name must have already been defined for the `attrData` object via `attrDefaults`. If missing, return a list of all attributes for the specified nodes.
`value`	An R object to set as the attribute value for the specified items. If the object has length one or does not have a length method defined, it will be assigned to all items in `x`. If the length of `value` is the same as `x`, the corresponding elements will be assigned. We will add an argument to indicate that the `value` is to be taken as-is for those cases where the lengths are the same coincidentally.