Add Whittaker and Muntus (1970) Ionic Radii

[morganjwilliams]

In addition to the Shannon radii, we could add an option to pyrolite.geochem.ind.get_ionic_radii to return the Whittaker and Muntus radii, which are likely more suitable for silicate geochemistry.

Relevant References

Whittaker, E.J.W., Muntus, R. (1970). Ionic radii for use in geochemistry. Geochimica et Cosmochimica Acta 34, 945–956. doi: 10.1016/0016-7037(70)90077-3

[morganjwilliams]

This table of radii was added, and the feature branch merged into develop with 9620b81. See the API docs for pyrolite.geochem.ind.get_ionic_radii for more info.

I've also updated the relevant example to show how the optional argument can be used (e.g, source='whittaker') and compare the two sets of ionic radii (or three, given Shannon 1976 has two sets itself).

Thanks to @charlesll for the suggestion!