Add comparison operators to Reaction class

I added __eq__ and __ne__ operators to the Reaction class in
order to make comparison between class instances possible. A reaction B
is considered to be equal to A, if and only if its reactants and
products are subsets of reaction A's reactants and products.
Do not include not recreated published reactions

Previous version was including *all* published reaction from the
database to the final dataset, even those which was not or cannot be
recreated by the available transforms, e.g. multi-product reactions. It
violated the assumption that the published reactions are a subset of
possible reactions generated by the transforms.

I modified the code to make it exclude published reactions not recreated
by the transform from the reaction list for which the descriptors are
calculated.
Collect and display some reaction statistics

I made the script to collect relevant data and display some rudimentary
statistics regarding both published and unpublished reactions as its execution
summary.