Get stiffness tensor defaults from materials HDF5

[cjh1]

@saransh13 Is going to look at added them the materials file.

[cjh1]

See #655

[joelvbernier]

@saransh13 -- nudging you on this. Maybe we just use some cubic reference? Steel, perhaps?

[saransh13]

Is there a good reference with these values compiled for different system? PR 144 on hexrd adds the ability to perform I/O already. Just need some values for the default materials.h5 file.