This fixes a similar deprecation warning on various pieces of code:
> DeprecationWarning: Testing an element's truth value will always return True in future versions. Use specific 'len(elem)' or 'elem is not None' test instead.
> samples = log.find ("samples") or empty_element
Add a new performance-log-progressive-coalesce.py tool, which
coalesces partial address maps in progressive performacne logs into
a single global address map, suitable for processing by the rest of
the tools.
Use the new tool as part of the pipeline in performance-log-viewer.
