Add RegulatoryNetwork behavior stub and conditional include

Add an empty RegulatoryNetwork behavior class (stub) that subclasses
Behavior. The header is included in biodynamo.h only when USE_BOOST is
defined. Implementation of Run() is left for follow-up work.

Author: sportokalidis

src/biodynamo.h

@@ -31,6 +31,9 @@
 #include "core/behavior/gene_regulation.h"
 #include "core/behavior/growth_division.h"
 #include "core/behavior/secretion.h"
+#ifdef USE_BOOST
+#include "core/behavior/regulatory_network.h"
+#endif
 #include "core/behavior/stateless_behavior.h"
 #include "core/environment/environment.h"
 #include "core/execution_context/copy_execution_context.h"

src/core/behavior/regulatory_network.h

@@ -0,0 +1,38 @@
+// -----------------------------------------------------------------------------
+//
+// Copyright (C) 2021 CERN & University of Surrey for the benefit of the
+// BioDynaMo collaboration. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+//
+// See the LICENSE file distributed with this work for details.
+// See the NOTICE file distributed with this work for additional information
+// regarding copyright ownership.
+//
+// -----------------------------------------------------------------------------
+
+#ifndef CORE_BEHAVIOR_REGULATORY_NETWORK_H_
+#define CORE_BEHAVIOR_REGULATORY_NETWORK_H_
+
+#include "core/behavior/behavior.h"
+
+#include "boost/numeric/odeint.hpp"
+#include "boost/phoenix/core.hpp"
+#include "boost/phoenix/operator.hpp"
+
+namespace bdm {
+class RegulatoryNetwork : public Behavior {
+  BDM_BEHAVIOR_HEADER(RegulatoryNetwork, Behavior, 1);
+
+ public:
+  RegulatoryNetwork() { AlwaysCopyToNew(); }
+  virtual ~RegulatoryNetwork() = default;
+
+  void Run(Agent* agent) override {};
+
+};
+}
+
+#endif  // CORE_BEHAVIOR_REGULATORY_NETWORK_H_
+