Creates a compound Attribute node. This function is used to read nodes which are not simple atomic types. It is designed to hold the Attribute in the form of unsigned char* along with information on how to interpret its contents.
Arguments:
attrname: String containing the Attribute's name.
offset: The byte offset in the data where the Attribute's value starts.
format: An atomic type, in character format, describing the Attribute, for example ``short'', or ``double''.
ndims: Number of dimensions in the Attribute's array.
dims: The dimensions of each of ndims.
Returns the compound node if successful or NULL upon failure.