fix up `effio summary` to print for humans

JSON will pretty much always require scrolling with the default
MarshalIndent style. I may add a -json flag at some point to print
json instead for use with other tools.
update comments
Rewrite summarization to be correct for ddirs.

Discovered a few bugs that I masked earlier by returning slices
from the subsamples/histograms.

This is now correct but a little slower since it's doing ~3 passes
over the data. The 3rd pass is sorting a []float of just the values
so not quite a full pass like the other two.
Remove debug printfs
fix up formatting for `effio summarize`