Refactor and add more materials

[markbandstra]

This PR will add additional materials commonly used in radiation modeling from the following reference:

McConn, Gesh, Pagh, Rucker, & Williams, "Compendium of Material Composition Data for Radiation Transport Modeling", PIET-43741-TM-963, PNNL-15870 Rev. 1

which is available here.

Detwiler, Rebecca S., McConn, Ronald J., Grimes, Thomas F., Upton, Scott A., & Engel, Eric J., "Compendium of Material Composition Data for Radiation Transport Modeling". United States. PNNL-15870 Revision 2.

which is available in PDF and JSON form here.

The additional materials from this document will require an optional dependency on PyPDF2 to parse the data from the PDF document.

Additional features in this PR:

• Materials data will be cached locally in a simple CSV format, so it only needs to be retrieved once. Currently, internet access is required every time one wishes to access the NIST material data.
• Materials data will be refactored so that it is available as a dict of dicts, making access to each individual material's data easier than the current pandas.DataFrame approach.

Items to complete:

• [x] Raise warning if Compendium data cannot be parsed, provide details on how to access it
• [ ] Update README to explain optional dependency
• [x] Make test coverage more complete
• [x] Provide option for clearing materials cache
• [x] Update to new 2021 version of the Compendium
• [x] Remove debugging outputs
• [x] Update notebooks

[markbandstra]

@jvavrek helpfully noted that there is a new revision of the Compendium available, and it is even available on a website for download as JSON. This will make this PR a bit easier and eliminate the need for parsing the PDF.