Add in parsing methods for accessing information about raw document

[eflumerf]

https://cdcvs.fnal.gov/redmine/projects/artdaq-utilities/repository/database/revisions/develop/entry/built-in/fhicl-cpp-v4_15_01.patch

In the artdaq_database product, we use a modified version of the fhicl-cpp library that allows us to capture information about the document before #includes are parsed (among other things). We have recently been having issues with symbols from the artdaq_database libraries conflicting with those defined by the fhiclcpp.so library, and feel that the best solution would be to integrate our changes into fhiclcpp directly (changing our extended_value to extended_extended_value, and parse_document to partially_parse_document, etc).