Add support for removeZeroSeries function

[carrieedwards]

This PR adds support for removeZeroSeries in Graphite web, which operates similarly to removeEmptySeries, but instead checks data points in the series for values equal to 0, instead of values equal to math.NaN().

[shanson7]

I don't see this function in graphite docs. Metrictank functions should also be in graphite.

[ywwg]

This function is in go-graphite, which a customer is using so we need to support this function.

[ywwg]

looks good, but I would like to see the empty series case tested

[CLAassistant]

All committers have signed the CLA.

[Jxlam-pdx]

@replay, this PR is ready to go. @carrieedwards is not authorized to merge it. Can you grant her access? thanks!

[replay]

The PR should also update the table of functions here: https://github.com/grafana/metrictank/blob/master/docs/graphite.md#processing-functions

As Sean pointed out, this function doesn't exist in Graphite. How would you feel about also contributing it to Graphite after merging this PR, for parity?

[carrieedwards]

I can contribute the same to Graphite as well.

[replay]

I can contribute the same to Graphite as well.

that would be great, thank you