`minimum` and etc. now work on PiecewiseSpace Funs

before couldn't do

```julia
julia> using ApproxFun

julia> x = Fun(identity,Interval(0.,10.))
Fun{UltrasphericalSpace{0},Float64}([5.0,5.0],UltrasphericalSpace{0}(Interval{Float64}(0.0,10.0)))

julia> minimum(abs(sin(x)))
-1.3112493384984874e-16
```