relax type definition of middle

this adds support for computing the median of unitful types, see PainterQubits/Unitful.jl#202

src/Statistics.jl

@@ -713,15 +713,15 @@ Compute the middle of a scalar value, which is equivalent to `x` itself, but of
 middle(x::Union{Bool,Int8,Int16,Int32,Int64,Int128,UInt8,UInt16,UInt32,UInt64,UInt128}) = Float64(x)
 # Specialized functions for real types allow for improved performance
 middle(x::AbstractFloat) = x
-middle(x::Real) = (x + zero(x)) / 1
+middle(x::Number) = (x + zero(x)) / 1
 
 """
     middle(x, y)
 
 Compute the middle of two reals `x` and `y`, which is
 equivalent in both value and type to computing their mean (`(x + y) / 2`).
 """
-middle(x::Real, y::Real) = x/2 + y/2
+middle(x::Number, y::Number) = x/2 + y/2
 
 """
     middle(range)