Correct earth radius value in @turf/helpers

[stebogit]

@DenisCarriere @rowanwins I noticed @turf/helpers still uses an incorrect earth radius, and I don't know why exactly since it was decided to change it: #635 (comment), #645

I think we should modify it, knowing it will probably be very tricky because impacting a lot of modules.

I'd also suggest we add an earthRadius function (just returning 6371008.8, meters) in @turf/helpers so we can refer to it in all the modules that right now need that value (I see the Rhumb modules and @turf/buffer for example), or in future modules, to avoid mismatching.

[DenisCarriere]

Yep agreed, we don't necessarily need a function, we just need to export variables like this library: https://github.com/mapbox/wgs84/blob/master/index.js

We should expose those 4 variables in turf helpers

[stebogit]

Oh cool, didn't know about exposing variables like this, but it makes sense. 👍

[stebogit]

Fixed with #980