Basic flow of data through the system.

The data can now flow into a graph, follow its path on inptus/outputs and be fetched from the graph after execution.
Currently graphs are executed synchronous.

lib/pls/include/pls/dataflow/input.h

+
+#ifndef PLS_DATAFLOW_INPUT_H_
+#define PLS_DATAFLOW_INPUT_H_
+
+#include "pls/dataflow/internal/input.h"
+
+namespace pls {
+namespace dataflow {
+
+template<int P, typename T>
+class input {
+  template<int OP, typename OT>
+  friend
+  class output;
+
+  using internal_type = internal::input<P, T>;
+  internal_type &internal_input_;
+
+ public:
+  explicit input(internal_type &internal_input) : internal_input_{internal_input} {};
+};
+
+}
+}
+
+#endif //PLS_DATAFLOW_INPUT_H_

lib/pls/include/pls/dataflow/output.h

+
+#ifndef PLS_DATAFLOW_OUTPUT_H_
+#define PLS_DATAFLOW_OUTPUT_H_
+
+#include "input.h"
+
+#include "pls/dataflow/internal/output.h"
+
+namespace pls {
+namespace dataflow {
+
+template<int P, typename T>
+class output {
+  using internal_type = internal::output<P, T>;
+  internal_type &internal_output_;
+
+ public:
+  explicit output(internal_type &internal_output) : internal_output_{internal_output} {};
+
+  void operator>>(const input<P, T> &input) {
+    internal_output_.connect(input.internal_input_);
+  }
+};
+
+}
+}
+
+#endif //PLS_DATAFLOW_OUTPUT_H_