Adds new absmax aggregation method

[yadsirhc]

absmax is a new aggregation method which returns the largest
absolute value when aggregating to a lower precision archive.

Useful for data such as time offsets, where you care about retaining
the value furthest from zero when aggregating but would prefer to
preserve whether the offset was positive or negative.

[steve-dave]

@yadsirhc Does having a absmin function make any sense at all? If so, perhaps package it in with this PR?

[yadsirhc]

I have yet to encounter a use case firsthand where an absmin function would make sense. I did consider adding one for the sake of symmetry, but soon realized that it would not be able to leverage built-in Python aggregation functions, so it felt like overkill.

[steve-dave]

@yadsirhc Python is not my expertise, but would this be too inefficient:

return min(knownValues, key=abs)

If Python can do that efficiently then it might also be possible to simplify your implementation of absmax with:

return max(knownValues, key=abs)

It all depends on how efficiently Python implements abs.

[yadsirhc]

Thanks for the suggestion. For small arrays it appears to be a negligible performance hit (~50 ns) for absmax versus the prior commit, but the code is much cleaner. In the case of absmin, I doubt another reimplementation could beat this one.

[steve-dave]

@esc does this look mergeable to you?

[SEJeff]

@yadsirhc can you please rebase this to the latest changes in whisper.git master?

[yadsirhc]

@SEJeff Done. Also updated the README to reflect avg_zero since it was the reason for the conflicts but hadn't itself been documented.

[deniszh]

@yadsirhc - could you please rebase your change if still needed?

[yadsirhc]

Rebased, as requested.

[coveralls]

Coverage decreased (-0.2%) to 88.83% when pulling 1fac0ff on yadsirhc:master into 1e96c0c on graphite-project:master.

[yadsirhc]

No conflicts remain with the base branch. Is the rebaseneeded label still appropriate? What is preventing merge now (or back in Aug 2016, for that matter)?

[deniszh]

Hello @yadsirhc,
Sorry for very long merging time, we're trying to maintain Graphite properly now.
Will merge this today or tomorrow, no worries.
Does anyone want to review too? /cc @graphite-project/committers