Tests whether a 3D line geometry has a constant height along its length.
An optional tolerance may be supplied for use when comparing the heights of vertices on the line. The line is considered to be level if the maximum height minus the minimum height along the line is less than or equal to the specified tolerance.
Parameter 1 - a 3D line geometry to be tested.
Parameter etc. (Optional) - a numerical value specifying the tolerance to apply to the line's z values.
Returns: a Boolean value, true if the geometry is a 3D line is level and false otherwise. Returns null if parameter 1 is not a 3D simple line geometry.