Improve handling of shear mixing's dependency on background mixing

[mnlevy1981]

Shear mixing needs two background mixing parameters for Pacanowski-Philander. There are three options for how to handle this:

1. Add background parameters to the shear mixing parameter type
2. Add a variable of background parameter type to the shear mixing parameter type
3. Have the user set a background parameter variable type and pass it to the shear module when using PP

Currently option (3) is in use, but I think it's the worst option of the group. I'm not sure which of the first two would be the best to use, though. I'm leaning towards (2) because that already has all the needed support for 1D or 2D variations in the background field.

[mnlevy1981]

Ended up adding PP_nu_b and PP_kappa_b to the cvmix_shear_params_type and removing the cvmix_background dependence altogether.