Replace comparison operators by dedicated method

The way we consider reactions to be equal -- one reaction's
reactants/products are to be others subsets -- is NOT symmetric
relation. It depends on order of the operands. Thus, using '==' operator
for checking it could be counterintuitive and leads to errors as one
usually expected that the order of such a comparison does not depend on
arguments order. Thus I replaced the operators with dedicated method
'equals' which clearly indicates which set inclusions we are interested
in.
Merge remote-tracking branch 'origin'

Resolved confilits of the remote branch with my local changes.
Change the default value of reaction year

Default value of the 'year' option of Reaction class was set to 2020.
This was not an optimal choice to indicate that the year was not set to
a given instance. I change it to None. This indicates more clearly the
intention.