Release 0.3 - Adds isomorphism checking

This release introduces a new module, called graphs-derived, which is meant to handle graph operations that derived or built upon the primitives provided by the "graphs" module.

The first feature added to the graphs-derived module are procedures which allow users to check for graph/subgraph isomorphism using the VF2 algorithm. This is an important step in that isomorphism is an important property in many graph-theoretic applications.

Finally, this release manages to fix a few bugs present in Release 0.2, mostly relating to edge cases or small typos within the code.