add default value of Earth's radius to Haversine

I don't think it should be necessary to look up the radius of Earth every time one wants to use the Haversine formula, so I've added the equatorial radius of Earth in meters as a default. I hope most imperial unit users will agree that metric is a reasonable default for scientific libraries. Of course Earth has different radii depending on where it is measured, as it isn't a perfect sphere. Uses sensitive to this should use more specific values. FYI this same default value is used in R. source https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html

src/haversine.jl

@@ -9,6 +9,7 @@ The computed distance has the same units as that of the radius.
 struct Haversine{T<:Real} <: Metric
     radius::T
 end
+Haversine() = Haversine(6378137.0)
 
 const VecOrLengthTwoTuple{T} = Union{AbstractVector{T}, NTuple{2, T}}
 
@@ -33,6 +34,6 @@ function (dist::Haversine)(x::VecOrLengthTwoTuple, y::VecOrLengthTwoTuple)
     2 * dist.radius * asin( min(√a, one(a)) ) # take care of floating point errors
 end
 
-haversine(x::VecOrLengthTwoTuple, y::VecOrLengthTwoTuple, radius::Real) = Haversine(radius)(x, y)
+haversine(x::VecOrLengthTwoTuple, y::VecOrLengthTwoTuple, radius::Real = 6378137.0) = Haversine(radius)(x, y)
 
 @noinline haversine_error() = throw(ArgumentError("expected both inputs to have length 2 in Haversine distance"))