Add estimate_parameters to the voigt component

Basically copied the estimate_parameters from the gaussian
component
Remove direct estimation of parameters in ComponentFit

Rather ComponentFit checks the component if it has an estimate_parameters
function, and use that function instead.
Component arctan, rename centre to x0, remove y0, add minimal_at_zero

minimal_at_zero adds an pi/2 arctan before the amplitude.
This basically shifts the minimum of the arctan component to zero.