A direct subclass of SimpleList, having no extra slots, in which listData is a list of position frequency matrices (PFMs), and the elementMetadata slot is a DataFrame with fifteen columns describing each matrix. Upon loading the MotifDb class, one MotifList object is instantiated and filled with matrices and their metadata. There should be no need for users to explicitly create objects of this class. When you load the MotifDb package, a fully-populated instance of this class is created, with > 2000 matrices with metadata
subset(x): extract matrices by metadata.
export(x): write matrices
show(x): describe matrices compactly
query(x): find matrices