#35 speed up cloning

[Stefan Mueller]

Bob Weigel wrote:
> I installed the NetBeans profiler and found that almost all of the time
> is being spent in MathLib.Interpreter.RootObject.clone. The amount of
> time spent doing the sum is about 0.015 seconds. The time spent cloning
> is about 1.3 seconds. In Octave on Cygwin, tic,sum(A),toc takes 0.12
> seconds.