Expose calc_geomean function

Expose calc_geomean to other packages so that it can used when plotting graph.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@216545 91177308-0d34-0410-b5e6-96231b3b80d8
Teach LNT to plot geomean trend

Public perf server has moved to PostgreSQL, we no longer suffer from
performance issue. It makes heavy jobs like plotting geomean trend possible.

This patch separates some statistics logic and teaches LNT to plot geomean.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@216546 91177308-0d34-0410-b5e6-96231b3b80d8
Label the test name if there are more than one plot

This helps distingushing data points when plotting multiple graphs, especially
when data points on not on the trend line, for example show all points.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@216547 91177308-0d34-0410-b5e6-96231b3b80d8