Optional extra stops to refine extent transforms

Add optional extra stops to refine some non-parallel transforms

Refine extent transforms use by Gratucule by using 32 points (8 per side) instead of the standard corners only transform
Update Graticule class description with current limitations

Correct projection extent in Mollweide example and revise opening zoom level correspondingly

examples/sphere-mollweide.js

@@ -17,8 +17,8 @@ register(proj4);
 // and a world extent. These are required for the Graticule.
 const sphereMollweideProjection = new Projection({
   code: 'ESRI:53009',
-  extent: [-9009954.605703328, -9009954.605703328,
-    9009954.605703328, 9009954.605703328],
+  extent: [-18019909.21177587, -9009954.605703328,
+    18019909.21177587, 9009954.605703328],
   worldExtent: [-179, -89.99, 179, 89.99]
 });
 
@@ -37,6 +37,6 @@ const map = new Map({
   view: new View({
     center: [0, 0],
     projection: sphereMollweideProjection,
-    zoom: 0
+    zoom: 1
   })
 });