Define and export Heaviside function

[dpsanders]

julia> function heaviside(x::Interval)
           if sup(x) > 0
               if inf(x) > 0 return 1..1
               else return 0..1
               end
           end
           return 0..0
       end

[OlivierHnt]

Of course one would have to also change the decoration at the jump point 0.

[dpsanders]

Superseded by #655