Added a function to numerically calculate derivative
Removed falsePosition and secant methods

Ridder is better than both of them and I wasn't getting falsePosition to
work anyway.

Also fixed a problem with the calculating of precision, previously if the
desired root was at x=0, or something very near it, the calculated
precision would have been very very small, or sometimes 0.
Completed newton and simpNewton

simpNewton automatically calculates the derivative using the numDeriv
function