Class that describes how the value of a given attribute should be retrieved.

The class contains the following members:

• name: the name of the attribute. This is also used when we are trying to get its value from a vertex/edge attribute of a graph.
• alt_name: alternative name of the attribute. This is used when we are trying to get its value from a Python dict or an igraph.Configuration object. If omitted at construction time, it will be equal to name.
• default: the default value of the attribute when none of the sources we try can provide a meaningful value.
• transform: optional transformation to be performed on the attribute value. If None or omitted, it defaults to the type of the default value.
• func: when given, this function will be called with an index in order to derive the value of the attribute.