Class RDFReader

Reader for RDF reaction files.  
OEChem supposedly supports reading these already, in terms of listing 
out all of the molecules from the reactions, but it loses important information.  
- It misses the very first molecule of the first reaction completely,
- It doesn't keep the reaction components together, so it's unclear 
    where one reaction begins and another ends, as well as
    being unclear whether each molecule encountered is a reactant or product.
- Data annotations applied to the reaction as a whole are missed.

Given an RDF file as text input, will generate an iterator over
the reactions in the file with a single composite reaction molecule
object representing each reaction, not each component.  Furthermore,
each of these "reaction molecules" will include all of the relevant
annotations as SD data pairs.

Probably don't need to instantiate this directly, use reactionmolistream instead,
which takes a filename and figures out whether to use this wrapper or just
a standard oemolistream based on the filename extension.