Use broadcasting for mid of IntervalBox (#211)

Author: dpsanders

src/multidim/intervalbox.jl

@@ -50,8 +50,8 @@ Return a vector of the `mid` of each interval composing the `IntervalBox`.
 
 See `mid(X::Interval, α=0.5)` for more informations.
 """
-mid(X::IntervalBox) = mid.(X.v)
-mid(X::IntervalBox, α) = mid.(X.v, α)
+mid(X::IntervalBox) = mid.(X)
+mid(X::IntervalBox, α) = mid.(X, α)
 
 big(X::IntervalBox) = big.(X)