rename lazyderivative and friends to lazy_derivative

[mohamed82008]

Following @odow's comment in #5 (comment), I think it's best to rename lazyderivative and friends to lazy_derivative and the likes to match the naming convention in the rest of the package.

[odow]

Others would be

AbstractDifferentiation.jl/src/AbstractDifferentiation.jl

Line 17 in 4aca738

reduceorder(b::AbstractBackend) = b

AbstractDifferentiation.jl/src/AbstractDifferentiation.jl

Line 27 in 4aca738

secondlowest(b::AbstractBackend) = b

AbstractDifferentiation.jl/src/AbstractDifferentiation.jl

Line 32 in 4aca738

primalvalue(::AbstractFiniteDifference, ::Any, f, xs) = f(xs...)

AbstractDifferentiation.jl/src/AbstractDifferentiation.jl

Line 471 in 4aca738

function lazygradient(ab::AbstractBackend, f, xs...)

AbstractDifferentiation.jl/src/AbstractDifferentiation.jl

Line 474 in 4aca738

function lazyhessian(ab::AbstractBackend, f, xs...)

AbstractDifferentiation.jl/src/AbstractDifferentiation.jl

Line 477 in 4aca738

function lazyjacobian(ab::AbstractBackend, f, xs...)

I also don't know what these one-letter structs are

AbstractDifferentiation.jl/src/AbstractDifferentiation.jl

Line 481 in 4aca738

struct D{B, F}

AbstractDifferentiation.jl/src/AbstractDifferentiation.jl

Line 495 in 4aca738

struct H{B, F}

[gdalle]

This is solved by #9