Write out ranges of entities in the RoboCORD output

[gaurav]

To help downstream users figure out the context of the entity match, we should include two pieces of information in the output:

• The start and end value of the matched entity in the input string, so we can check whether two entities were close together or not
• The section heading where the entity was found. Some sections might represent more important relationships (e.g. methods, results, discussion, conclusions) than others (introduction).

We could also include the matched string from the paper if that would be helpful.