Add "toplev.py"

toplev.py is a tool to do cycle composition using the "top down
methology" described in the Intel Optimization manual

Do cycle decomposition on a workload: estimate on which part of the
CPU pipeline it bottlenecks. The bottlenecks are expressed as a tree
with different levels (max 4).

Requires an Intel Sandy, Ivy Bridge, Haswell CPU.
It works best on Ivy Bridge currently, the others only support
a basic (but reliable) model.
describe toplev.py in README.md