Add aggregation type support

[ciaranj]

Based on top of the work done by @cwholt further fixes and an implementation of the aggregation method support, so now when aggregating to lower precision archives 'max, min, sum, last and average' can be used instead of just always averaging (the same as python's whisper)

[ciaranj]

Be aware of @3c8b006 that might be contentious!

[jnovack]

3c8b006 is "contentious" but needed for compatibility. I like it. Let's get it merged!

[ciaranj]

Just so you're aware, I've stopped working on this library, and moved over to https://github.com/ciaranj/ceres/tree/nodejs_port might be of use :)