create and export isconstant

Author: vpuri3

src/SciMLOperators.jl

@@ -56,6 +56,8 @@ export ScalarOperator,
 export update_coefficients!,
        update_coefficients,
 
+       isconstant,
+
        cache_operator,
 
        has_adjoint,

src/interface.jl

@@ -93,9 +93,18 @@ has_ldiv!(L::AbstractSciMLOperator) = false # ldiv!(du, L, u)
 
 ### Extra standard assumptions
 
-isconstant(L) = true
+isconstant(::Union{
+                   # LinearAlgebra
+                   AbstractMatrix,
+                   UniformScaling,
+                   Factorization,
+
+                   # Base
+                   Number,
+
+                  }
+          ) = true
 isconstant(L::AbstractSciMLOperator) = all(isconstant, getops(L))
-#isconstant(L::AbstractSciMLOperator) = L.update_func = DEFAULT_UPDATE_FUNC
 
 islinear(L) = false
 islinear(::Union{