Backport: Implement calculus operations for ConstantSpace (#597)

Author: jishnub

src/Spaces/ConstantSpace.jl

@@ -254,7 +254,7 @@ choosedomainspace(M::ConcreteMultiplication{D,UnsetSpace},sp::Space) where {D<:C
 Base.isfinite(f::Fun{CS}) where {CS<:ConstantSpace} = isfinite(Number(f))
 
 
+## Calculus
 
-
-
-
+integrate(f::Fun{<:ConstantSpace{<:IntervalOrSegment}}) = Number(f) * Fun(domain(f))
+differentiate(f::Fun{<:ConstantSpace{<:IntervalOrSegment}}) = zero(f)

test/SpacesTest.jl

@@ -309,6 +309,7 @@ using LinearAlgebra
         @test g > f
         @test g >= f
         @test 1 < f < 3
+        @test differentiate(f) == Fun(0, ConstantSpace(0..1))
 
         @test maxspace(ConstantSpace(Point(1)), ConstantSpace(Point(2))) == ConstantSpace(Point(1) ∪ Point(2))
         @test maxspace(ConstantSpace(Point(1)), ConstantSpace(AnyDomain())) == ConstantSpace(Point(1))