[Feature Req] Use nu = delta_c / sigma

[steven-murray]

Is your feature request related to a problem? Please describe.
Mathematically things make a bit more sense if nu is delta_c/sigma rather than its square.

Describe the solution you'd like
Change nu everywhere to be this, rather than squared.

Describe alternatives you've considered
This is problematic because it definitely changes the API. I'd have to add in some warnings, and actually implement the change in v4.0