Redefined functions relating to collective diffusion coefficient and reduced ionic condictivity.

[kitmccoll]

Left the notebook the same, since it will now be correct with the way the functions are defined.

[bjmorgan]

1. You have some rogue non-ascii characters in one of the docstrings.
2. I've been comparing these to the definitions in the LLZO paper. In that paper, the "reduced ionic conductivity" is defined as D_J * the site occupancy x of the lattice. What is currently calculated is proportional to this, but is not exactly the same (although units for these are somewhat arbitrary). I am wondering whether we want to add a scaling factor of N_sites, which would make the updated code consistent with Eqn. 3.3, but would rescale the results presented in the paper.
3. It might be nice to expand the docstrings so that these direct the reader to the relevant equations in the RSOS paper?