autodiff

It's used only as a dev dependency in 4 crates.

Number of dependers	autodiff version	Downloads/month
3	0.7.0	98
1	0.6.0	8

Depender (with downloads and own rev deps)			autodiff version
200		autodj	dev	^0.7.0
	2	meshx	dev	^0.7
		hrbf	dev	^0.7
		dfo	dev	^0.6