Add decay feature vector

This feature vector takes the principle that the influence of each
ring is related to some constant times a decay factor from the edge
of the chain.
Add centered decay feature vector

This is the same as the decay feature vector with the exception that
it does not work from the "starting" edge of the structure. It instead
looks at the center of the structure and does its decay functions from
there. Both sides are treated the same.
Add signed centered decay feature vector

This feature vector works the same as the centered decay feature vector
with the addition that it includes the side of the center that the rings
are on.
Improve documentation and formating
Move the decay function to the utils module
Add flip binary feature vector

This is the same as the normal binary feature vector with the addition
of an extra element for each triplet that corresponds to whether or not
the aryl group was flipped.
Adjust formatting of statistics