Add WGS84 to @turf/helpers

[DenisCarriere]

Add WGS84 to @turf/helpers

Pulled from @tmcw's https://github.com/mapbox/wgs84

Built-in variables for WGS84 related things.

Ref: #978

How to use

import { wgs84 } from '@turf/helpers';

wgs84.RADIUS // => 6378137

CC: @stebogit

[DenisCarriere]

@stebogit Weird errors like this happen... 🤔

https://travis-ci.org/Turfjs/turf/builds/280554132#L1495

lerna ERR! execute     if (mutate !== true) geojson = clone(geojson);
lerna ERR! execute                                    ^
lerna ERR! execute 
lerna ERR! execute TypeError: clone is not a function

I think these errors are caused by export default function() when the modules don't have a name and for some reason it can't find external functions 🤔

https://github.com/Turfjs/turf/blob/master/packages/turf-clone/index.js#L13

Going to merge this since 1/2 of the tests passed.

[stebogit]

@DenisCarriere the earth radius is not what here was agreed upon.
In my understanding it should be 6371008.8 meters.

[DenisCarriere]

Yep, I changed it to the value you provided, the earth radius value we need is for the Haversine area/distance calculations.

[DenisCarriere]

earthRadius => 6371008.8