Proposal for a better coarse mesh for 3d spherical shells for Earthlike ...

...aspect ratios

I have found that for Earthlike aspect ratios (R_inner/R_outer ~ 0.55) the starting mesh in 3D has pretty distorted cells, especially near the CMB.  This does not really get better on isotropic refinement.  I have a patch that does one step of anisotropic refinement for the starting mesh in order for the cells to be more cubelike.

It is a bit hacky at the moment, but I thought I'd bring it up to see if people think this would be a useful change for the 3D mesh.

I attach a before and after image for your perusal.

![mesh1](https://cloud.githubusercontent.com/assets/5728311/3529933/2f1abc72-079f-11e4-911f-a4b02daa6b32.png)
![mesh2](https://cloud.githubusercontent.com/assets/5728311/3529939/3494eb1e-079f-11e4-864c-48625fa78980.png)