Add convenience functions to create common materials

[KyleVaughn]

It's painful to create materials one isotope at a time, especially given how often some common materials are used. Therefore, we should mimic the Material.setUO2 routine, which uses the density of the material along with user-specified U-235 enrichment (wt%) and Gd2O3 concentration (wt%) to set the material's nuclides and number densities.

Add the following:

• SS304
• Zirc4
• H2O