Guard Boost includes in regulatory_network.h with USE_BOOST

rootcling (ROOT's dictionary generator) compiles headers independently
of the normal build, bypassing the USE_BOOST guard in biodynamo.h.
Wrapping the entire header content in #ifdef USE_BOOST makes the file
safe to process in any context when Boost is not present.

Author: sportokalidis

src/core/behavior/regulatory_network.h

@@ -15,6 +15,8 @@
 #ifndef CORE_BEHAVIOR_REGULATORY_NETWORK_H_
 #define CORE_BEHAVIOR_REGULATORY_NETWORK_H_
 
+#ifdef USE_BOOST
+
 #include "core/behavior/behavior.h"
 
 #include "boost/numeric/odeint.hpp"
@@ -34,5 +36,7 @@ class RegulatoryNetwork : public Behavior {
 };
 }
 
+#endif  // USE_BOOST
+
 #endif  // CORE_BEHAVIOR_REGULATORY_NETWORK_H_