Added overloaded atan

[dlamkins]

Lua 5.3 deprecated math.atan2(y, x). Most IDEs have begun showing a warning about this.

Lua 5.3 also added a new overload math.atan(y [, x]), which now accepts two parameters making it the intended replacement for math.atan2(y, x):

Deprecations in 5.3: https://www.lua.org/manual/5.3/manual.html#8.2 math.atan(y [, x]) in 5.3: http://www.lua.org/manual/5.3/manual.html#pdf-math.atan

I think normally it would make sense to provide a default value (shown below) to keep it a single function instead of requiring the overload, but because the existing parameter is named x, it would break any scripts which manually specify the parameter name.

public static double atan(double y, double x = 1)
            => Math.Atan2(y, x);

[neolithos]

Thank you.