Start analyzing depth profile: smoothing and time-based min/max/avg

This turns the depth profile into a generic "plot_info" and calculates
minima, maxima and averages over 1-, 2- and 3-minute intervals for each
point.  It also creates a smoothed version.

We currently don't actually show the results, but that's the next step..

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>