Restructure to allow a clean interface for end users.

We need some tricks in template programming to have a clean user facing API while internally using our classes with more capabilities.

lib/pls/include/pls/dataflow/internal/inputs.h

+
+#ifndef PLS_DATAFLOW_INTERNAL_INPUTS_H_
+#define PLS_DATAFLOW_INTERNAL_INPUTS_H_
+
+#include <tuple>
+
+#include "input.h"
+
+namespace pls {
+namespace dataflow {
+namespace internal {
+
+template<int P, typename ...I>
+class inputs {
+  using values_type = std::tuple<input<I>...>;
+  values_type values_;
+
+ public:
+  template<int N>
+  typename std::tuple_element<N, values_type>::type &get() {
+    return std::get<N>(values_);
+  }
+};
+
+}
+}
+}
+
+#endif //PLS_DATAFLOW_INTERNAL_INPUTS_H_

lib/pls/include/pls/dataflow/internal/outputs.h

+
+#ifndef PLS_DATAFLOW_INTERNAL_OUTPUTS_H_
+#define PLS_DATAFLOW_INTERNAL_OUTPUTS_H_
+
+#include <tuple>
+
+#include "output.h"
+
+namespace pls {
+namespace dataflow {
+namespace internal {
+
+template<int P, typename ...O>
+class outputs {
+  using values_type = std::tuple<output<O>...>;
+  values_type values_;
+
+ public:
+  template<int N>
+  typename std::tuple_element<N, values_type>::type &get() {
+    return std::get<N>(values_);
+  }
+};
+
+}
+}
+}
+
+#endif //PLS_DATAFLOW_INTERNAL_OUTPUTS_H_