Add Bondi and Alvarez Radii

[rsdefever]

Closes #10.

Adds van der Waals radii with two options, the Bondi radii and Alvarez radii.

Links to papers:
Alvarez (Table 1)
Bondi 1 (Tables I and XIV) and 2 (Table 1).

[jennyfothergill]

LGTM! I only found two differences and they had to do with the precision. Thanks for doing this. :)

[justinGilmer]

lgtm, just a few edits/double checks needed.

[rsdefever]

@justinGilmer can you confirm that the numbers were correct?

[justinGilmer]

Ok those numbers for Cd and Hg are correct in the json file. I was not using the updated values from the 2nd Bondi paper.