Map level of education to more descriptive constants

Note that this also groups the various codes for doctorate degrees together.

@brianhw @rocha 
So we would not have a single location?  This would re-create the file in each partition.  An alternative is to write it to the table_location instead.  I guess it depends on how often we expect to change the mapping stored in this file.  It's a tradeoff -- if we change the mapping in code, we would have to delete the data file manually.  Part of the motivation for the question is whether this table would be used for other ad hoc BI queries.  Seems that a table location is more accessible than multiple partition locations.  But I don't really understand the requirements for this BI queries.