DECKGL integration - Phase 1 (#3771)

* DECKGL integration

Adding a new set of geospatial visualizations building on top of the
awesome deck.gl library. https://github.com/uber/deck.gl

While the end goal it to expose all types of layers and let users bind
their data to control most props exposed by the deck.gl API, this
PR focusses on a first set of visualizations and props:

* ScatterLayer
* HexagonLayer
* GridLayer
* ScreenGridLayer

* Addressing comments

* lint

* Linting

* Addressing chri's comments

superset/assets/spec/javascripts/modules/geo_spec.jsx

@@ -0,0 +1,27 @@
+import { it, describe } from 'mocha';
+import { expect } from 'chai';
+
+import { unitToRadius } from '../../../javascripts/modules/geo';
+
+const METER_TO_MILE = 1609.34;
+
+describe('unitToRadius', () => {
+  it('converts to square meters', () => {
+    expect(unitToRadius('square_m', 4 * Math.PI)).to.equal(2);
+  });
+  it('converts to square meters', () => {
+    expect(unitToRadius('square_km', 25 * Math.PI)).to.equal(5000);
+  });
+  it('converts to radius meters', () => {
+    expect(unitToRadius('radius_m', 1000)).to.equal(1000);
+  });
+  it('converts to radius km', () => {
+    expect(unitToRadius('radius_km', 1)).to.equal(1000);
+  });
+  it('converts to radius miles', () => {
+    expect(unitToRadius('radius_miles', 1)).to.equal(METER_TO_MILE);
+  });
+  it('converts to square miles', () => {
+    expect(unitToRadius('square_miles', 25 * Math.PI)).to.equal(5000 * (METER_TO_MILE / 1000));
+  });
+});