Add aggregation function EXPERIMENTAL_PPROF

traceconv currently generates a pprof profile for all samples in a
trace. For some analysis we might be interested in only looking at
certain samples (e.g. those that intersect a given slice). This new
function allows to generate a pprof profile for a given set of samples.

E.g. To get a profile per thread

SELECT tid, EXPERIMENTAL_PPROF(callsite_id)
FROM perf_sample
JOIN thread USING (utid)
GROUP BY tid

Change-Id: I5cbb86248488a89a2bc683a6df9e5b24f923ddd3