A dictionary mapping the index of an attribute to a dictionary
that maps attribute names to either possible values (for
nominal variables) or to the strings 'numeric' or 'string' (for
those datatypes).
For example, if the first attribute line in the file was:
@attribute outlook {sunny, overcast, rainy}
Your attribute dictionary would contain an entry that looked like:
{1: {'outlook' : ['sunny','overcast','rainy']}}