Started working on volumetric features module.

Added some base functions for working with modules. It's not tested, but it compiles. CVARs are not created yet.
Implemented volume events.
Function VolUpdatePlayerChanges with help from VolGetPlayerStates detects if a player leave or enter a volume between main timer intervals. This solution is not final and might be expensive. Needs to be optimized later!

src/zombiereloaded.sp

@@ -57,6 +57,7 @@
 #include "zr/zhp"
 #include "zr/jumpboost"
 #include "zr/anticamp"
+#include "zr/volfeatures/volfeatures"
 
 // Almost replaced! :)
 #include "zr/zombie"