use isinteger in assert_isinteger (#299)

* use isinteger

* Relax Laplacian order

Author: jishnub

src/ApproxFunBase.jl

@@ -131,7 +131,7 @@ promote_eltypeof(As::Union{AbstractArray, Tuple}) = mapfoldl(eltype, promote_typ
 
 assert_integer(::Integer) = nothing
 function assert_integer(k::Number)
-    @assert Integer(k) == k "order must be an integer"
+    @assert isinteger(k) "order must be an integer"
     return nothing
 end
 

src/PDE/PDE.jl

@@ -9,7 +9,8 @@ lap(d::Domain) = Laplacian(d)
 lap(f::Fun) = Laplacian()*f
 
 
-function Laplacian(d::BivariateSpace,k::Integer)
+function Laplacian(d::BivariateSpace, k::Number)
+    assert_integer(k)
     Dx2=Derivative(d, SVector{2}(2,0))
     Dy2=Derivative(d, SVector{2}(0,2))
     if k==1