Exclude data records with missing article reference

[hlapp]

Now that available article references have been comprehensively curated for the combined dataset, data records with missing article references should probably be excluded from the analysis, Also, books and other types of references that will not be in Scopus (and for which citation data will thus not be found, erroneously suggesting zero citations), should probably also be excluded.

@lsheble to discuss with @bendmorris.

[hlapp]

@lsheble has created subsets of data, will put into repo with annotation.