Add constructor of matrix power given a target index (#132)

* add constructor of matrix power given a target index

* pull out constructors

src/power.jl

@@ -56,17 +56,26 @@ mutable struct IntervalMatrixPower{T}
     Mᵏ::IntervalMatrix{T}
     k::Int
 
-    function IntervalMatrixPower(M::IntervalMatrix{T}) where {T}
-        return new{T}(M, M, 1)
-    end
-
     function IntervalMatrixPower(M::IntervalMatrix{T}, Mᵏ::IntervalMatrix{T},
                                  k::Int) where {T}
         @assert k >= 1 "matrix powers must be positive"
         return new{T}(M, Mᵏ, k)
     end
 end
 
+function IntervalMatrixPower(M::IntervalMatrix{T}) where {T}
+    return IntervalMatrixPower(M, M, 1)
+end
+
+function IntervalMatrixPower(M::IntervalMatrix{T}, k::Int) where {T}
+    @assert k >= 1 "matrix powers must be positive"
+    pow = IntervalMatrixPower(M, M, 1)
+    @inbounds for i in 1:(k-1)
+        increment!(pow)
+    end
+    return pow
+end
+
 function copy(pow::IntervalMatrixPower)
     return IntervalMatrixPower(pow.M, pow.Mᵏ, pow.k)
 end