Option on DcmipInitialConditions unclear

[mcgibbon]

The DcmipInitialConditions object has an option add_perturbation with the docstring Whether a perturbation must be added. Only applies to the baroclinic wave test.. What does this mean? what does it mean for a perturbation to need to be added? A perturbation on the normal initial conditions? What kind of perturbation? We should clear up this description.

[mcgibbon]

This problem remains after the v0.15 update