Function generic_inverse_2d 

pub fn generic_inverse_2d<C: CoordinateStep, P: TransformCoordinates>(
    xy: &P,
    step: &C,
    lp: &mut P,
    delta_xy_tolerance: f64,
)

Compute (lam, phi) corresponding to input (xy.x, xy.y) for projection P.

Uses Newton-Raphson method, extended to 2D variables, that is using inversion of the Jacobian 2D matrix of partial derivatives. The derivatives are estimated numerically from the proj.fwd method evaluated at close points.

Note: thresholds used have been verified to work with adams_ws2 and wink2

Starts with initial guess provided by user in lp_initial