rename new_flow_logic subdirectory to a less ambiguous name

The "new flow logic" name was supposed to indicate that it was a continuation of the old branch by the same name, but it is beginning to become clear that it's not "new" any more and it might lead to confusion with "classic mode" flow logic while that still co-exists.
Explicitly name the subdirectory "pressure logic" to give a better idea of what goes in it, init.lua edited accordingly.

init.lua

@@ -126,7 +126,7 @@ dofile(pipeworks.modpath.."/filter-injector.lua")
 dofile(pipeworks.modpath.."/trashcan.lua")
 dofile(pipeworks.modpath.."/wielder.lua")
 
-local logicdir = "/new_flow_logic/"
+local logicdir = "/pressure_logic/"
 
 -- note that even with these files the new flow logic is not yet default.
 -- registration will take place but no actual ABMs/node logic will be installed,