Doen't seem to support GeometryCollection

Nobody has claimed this yet.

Leaflet-pip does not seem to find polygons inside of a GeometryCollection. If I get it right then not only mush layers be iterated, if the layer contains a GeometryCollection then all items within that must also be subjected to isPoly(l), but that probably require a bit of refactoring to process GeoJSON objects rather than layers.

So currently a Polygon as the one the below will never be found.
`
{
"type": "FeatureCollection",
"features": [{
"type": "Feature",
"properties": {
"class": "LYRESTAD"
},
"geometry": {
"type": "GeometryCollection",
"geometries": [{
"type": "Polygon",
"coordinates": [
[
[14.00881897363853, 58.759130043177166],
[13.928815204319006, 58.93581644468062],
[14.120750873038237, 58.8843341034047],
[14.121862332723687, 58.88126014491872],
[14.174039271588722, 58.78656397216331],
[14.145110948923927, 58.767173887639935],
[14.12508839353722, 58.758541330342304],
[14.11335950201629, 58.76108407639584],
[14.06140521182085, 58.77226018822536],
[14.00881897363853, 58.759130043177166]
]
]
}]
}
}
`

Start at the layer iteration and isPoly(l) path mentioned in the issue, then trace how GeoJSON objects are processed. Reproduce the supplied FeatureCollection with a GeometryCollection containing a Polygon; done means the nested Polygon is found by leaflet-pip.

Written by the indexing model from the issue text.