Pmidcite

* Corrected: [AttributeError: 'NoneType' object has no attribute 'pmid' 4](https://github.com/dvklopfenstein/pmidcite/issues/4)

* Simplify code by instantiating NIHiCiteDownloader directly

* Simplified the pmidcite library interface [Notebook updates](https://github.com/dvklopfenstein/pmidcite/issues/2#issuecomment-894760798)
* Improved speed for downloading/loading NIH citation data for large (e.g. greater than 80,000) sets of PMIDs

* NIH citation data is now downloaded for only researcher-specified PMIDs by default. [3](https://github.com/dvklopfenstein/pmidcite/issues/3)
* To download NIH citation details for citations/references of a researcher-specified PMID, use the icite arguments:
* -v (download NIH citation details for both citations and references of a researcher-specified PMID)
* -c (download NIH citation details for the citations of a researcher-specified PMID)
* -r (download NIH citation details for the references of a researcher-specified PMID)

* Add script, icite (like icite.py)

* Updated all notebooks to use the new customizable paper grouping using the NIH's co-citation network data [2](https://github.com/dvklopfenstein/pmidcite/issues/2)