Add indicator percentpop euclidean proximity to openspace

[weiqi-tori]

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• No, I haven't read it.

Is this PR related to a JIRA ticket?

• Yes, the URL of the associated JIRA ticket is https://gfw.atlassian.net/browse/CIF-XXX. The PR name follows the format [CIF-XXX] my subject.

• No, this is a documentation update. The PR name follows the format [DOCS] my subject.

What changes were proposed in this PR?

How was this patch tested?

Did this PR include necessary documentation updates?

• Yes, I am adding a new API feature.
• Yes, I have updated the documentation update.
• No, this PR does not affect any public API so no need to change the docs.

[weiqi-tori]

Updates:

1. added a sum() zonal stats function to layer class, so don't need to use mean*count;
2. fixed the way .buffer() was used in the osm layer;
3. updated the metrix script to pull the osm and world_pop layers.
   Now it returns the percent of pop within the open space buffered area.