Support the inclusion of Zoom level in resource requests

[panaaj]

Ref: SignalK/freeboard-sk#222

Currently Resource API does not support a zoom parameter.

Resource providers may return a large number of entries within a given area which, when displayed on a map can result in a cluttered display.

It would be useful if a zoom level could be provided in the request to allow the provider to further filter the entries returned.
It is recognised that zoom level is a concept that is abstract and will vary in implementation between clients & resource providers, but ultimately it is still a useful filter parameter to support.

Zoom level parameter would accept an integer >= 1 (e.g. zoom=4)

Example request:
/signalk/v2/api/resources/notes?position=[5.562490300291487,53.01296548279345]&distance=92000&zoom=9

[KvotheBloodless]

Zoom level is indeed abstract...

A better value could be a meters (real) per pixel float value. A bit more challenging to calculate no doubt, but at least it would be consistent.

Another option could be actual meters displayed on the diagonal. I think Google maps uses this.

[tkurki]

See https://wiki.openstreetmap.org/wiki/Zoom_levels - "number of tiles to cover whole earth" is well defined, but the others are projection dependent. But I think in practise it is still very much usable.