mark some fields that should not be changed as const (#10)

mark some fields that should not be changed as const

Author: baggepinnen

src/DiscretePIDs.jl

@@ -17,17 +17,17 @@ mutable struct DiscretePID{T} <: Function
     "Reset time"
     Tt::T 
     "Maximum derivative gain"
-    N::T 
+    const N::T 
     "Fraction of set point in prop. term"
     b::T 
     "Low output limit"
     umin::T 
     "High output limit"
     umax::T 
     "Sampling period"
-    Ts::T 
+    const Ts::T 
     bi::T
-    ar::T
+    const ar::T
     bd::T
     ad::T
     "Integral state"