This patch constitutes a slightly different and also simpler
approach to computing the performance index.

The hierarchical index structure is gone for now,
and the index is computed essentially like this:

1: Specify a base timestamp BT.
2: Specify a set of evaluation timestamps ET.
3: Specify a window size for median smoothing.
4: Specify a set of filters to effectively narrow
   down the set of result histories that may contribute
   to the index (there are currently five separate filters:
   metric, platform, host, branch, and test case.
   Zero or more values may be specified for each one).
5: Evaluate the index at a given evaluation timestamp ET(i) by
   taking the arithmetic mean of the differences between the
   values at ET(i) and BT for the individual result histories.
   Both values are median-smoothed using a right-aligned window.