Changed the tanh derivative from g/cosh(x)**2 to g*(1-tanh(x)**2)

[RmStorm]

This way ans can be reused as in g*(1-ans**2) which should be a little quicker since it omits the cosh call. But mainly I get overflow warnings because of the use of cosh. This new quicker derivative solves that.

[j-towns]

👍 I found this quite a lot quicker and hav included it in #312, but that probably won't be merged into master very soon so this pr is good in the mean time. You can do something similar for the derivative of tan and of sinh.

[j-towns]

It's tantalising for derivatives of sin and cos too because calculating sqrt(1 - x**2) is way quicker than calculating sin/cos (around 8x faster on my laptop), but you need to also calculate the correct sign, which slows things back down, at least in my implementation attempts.

[mattjj]

This is great, thanks!