Interfaces that define the different ways the appearance of elements can be transformed.

The purpose of transformers is to change the appearance of elements based on user parameters and column's values. There are three types of transformers: simple, ranking and partition. The simple transformer interface is defined in `org.gephi.appearance.spi.SimpleTransformer`. The ranking and partition transformers are both based on the specific value an element has in a given column. The ranking transformer work with numerical, sorted values (e.g. age, number of followers) and the partition transformer work with categorical values (e.g. community id, gender). The ranking transformer is defined in `org.gephi.appearance.spi.RankingTransformer` and the partition transformer in `org.gephi.appearance.spi.PartitionTransformer`. Then, transformers can be associated with user interfaces elements through the implementation of `org.gephi.appearance.spi.TransformerUI`.