Tests whether a 3D line geometry is digitised in a downhill direction.
An optional tolerance may be supplied to use when comparing the heights of vertices on the line. The line is considered to be downhill if each vertex is lower than all the preceding vertices, to within the specified tolerance.
Parameter 1 - a 3D line geometry to test.
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 that slopes downwards and false otherwise. Returns null if parameter 1 is not a 3D simple line geometry.