Preventing crash during obscure use of SceneCache ROP.

This crash happens when the root parm of hte ROP is pointing at a visible OBJ which is inside an invisible OBJ subnet.
Improving performance of SceneCache SOP.

8.0.0-b2 introduced some code which improved performance when loading large hierarchies for the first time. This unfortunately caused a regression in performance when animating these hierarchies. This commit brings the animated perforamnce back inline with 8.0.0-b1, while maintaining (if not improving) the on-load performance from 8.0.0-b2.

Here are a few stats from a particularly heavy asset:

Load first frame
b1 10m 13.2s
b2 3m 41.9s
b3 7.26s

+1 Frame
b1 3.89s
b2 5m 17.9s
b3 5.1s

And here are a few stats from a standard character:

Load first frame
b1 1.35s
b2 0.28s
b3 0.48s

+1 Frame
b1 0.2s
b2 0.44s
b3 0.2s
SceneCache SOP sorts the childNames before recursing.

This guarantees shape order (and therefor point order) across multiple Houdini sessions. Without this sorting, the children would be created in InternedString-sorted order, which is constant within a single process, but is essentially random across processes.
Adding todo about sorting by string value in the first place
adding some comments
Merge pull request #176 from andrewkaufman/sccHoudiniPerformance

SceneCache SOP Performance and Consistancy
clarifying reRoot logic
Merge pull request #175 from andrewkaufman/ropCrashOnHidden

Preventing crash during obscure use of SceneCache ROP