Documentation of varscale, lagscale and conscale

[amontoison]

The functions varscale, lagscale and conscale are exported but they don't have any docstring.
It's fine for me only if the functions are not exported.
We should not have this kind of thing in the public API.

function varscale end
function lagscale end
function conscale end

[tmigot]

@dpo Can we remove those functions? Or what was the motivation?

[dpo]

They are inspired by the AMPL functions of the same name, and should help with problem scaling.