Ensure tree construction code is only compiled once,

[dehann]

Compiling should not repeat each time the graph changes and a new tree is constructed. First reported in:

• Is there any example for mult-architecture support [JuliaGPU] Caesar.jl#835

[Affie]

Compiling should not repeat each time the graph changes and a new tree is constructed

We have a trade-off between compile and run time. I am seeing the long compile times in parametric as well. I suspect it has something to do with factors as even addFactor takes long.

[dehann]

Clearly some bug or quirk here that's worth figuring out.

I wonder if it has something to do with current attempts to compute numerical gradients for factors in ccw.