added valid gettor

splinterpolator.h

@@ -127,6 +127,7 @@ public:
   //
   unsigned int NDim() const { return(_ndim); }
   unsigned int Order() const { return(_order); }
+  const bool Valid() const { return(_valid); }
   ExtrapolationType Extrapolation(unsigned int dim) const 
   { 
     if (dim >= _ndim) throw SplinterpolatorException("Extrapolation: Invalid dimension");