Various field/belt related stuff

- Remove all field booleans except quickfire and belt, which have been moved to cCurTown
- Alter and extend place_spell_pattern, to allow arbitrary damage types and to make it more clear in the code what's happening when it's called
- Delete fields.cpp file; a few things moved to locutils.cpp, but most are now part of cCurTown
- set_terrain function automatically updates belt present boolean if setting to a conveyor.

osx/boe.fileio.h

@@ -34,7 +34,6 @@ void import_template_terrain();
 void import_anim_terrain(short mode);
 void start_data_dump();
 void end_data_dump();
-void set_terrain(location l, ter_num_t terrain_type);
 //void oops_error(short error);
 //void set_up_ter_pics();
 //bool load_scenario();