vpslot_terrain_transform optional/mandatory

optional/mandatory have better support for extended JSON features, and
this cleans up the loading code.

As with the other slots, this slot doesn't really need a flag to specify
it, it could just use the existance of a transform_terrain member.

Author: ehughsbaird

src/veh_type.cpp

@@ -400,16 +400,7 @@ void vpart_info::load( const JsonObject &jo, const std::string &src )
         if( !transform_terrain_info ) {
             transform_terrain_info.emplace();
         }
-        JsonObject jttd = jo.get_object( "transform_terrain" );
-        vpslot_terrain_transform &vtt = *transform_terrain_info;
-        optional( jttd, was_loaded, "pre_flags", vtt.pre_flags, {} );
-        optional( jttd, was_loaded, "post_terrain", vtt.post_terrain );
-        optional( jttd, was_loaded, "post_furniture", vtt.post_furniture );
-        if( jttd.has_string( "post_field" ) ) {
-            mandatory( jttd, was_loaded, "post_field", vtt.post_field );
-            mandatory( jttd, was_loaded, "post_field_intensity", vtt.post_field_intensity );
-            mandatory( jttd, was_loaded, "post_field_age", vtt.post_field_age );
-        }
+        mandatory( jo, was_loaded, "transform_terrain", transform_terrain_info );
     }
 }
 
@@ -473,6 +464,18 @@ void vpslot_toolkit::deserialize( const JsonObject &jo )
     was_loaded = true;
 }
 
+void vpslot_terrain_transform::deserialize( const JsonObject &jo )
+{
+    optional( jo, false, "pre_flags", pre_flags, {} );
+    optional( jo, false, "post_terrain", post_terrain );
+    optional( jo, false, "post_furniture", post_furniture );
+    if( jo.has_string( "post_field" ) ) {
+        mandatory( jo, false, "post_field", post_field );
+        mandatory( jo, false, "post_field_intensity", post_field_intensity );
+        mandatory( jo, false, "post_field_age", post_field_age );
+    }
+}
+
 void vpart_info::set_flag( const std::string &flag )
 {
     flags.insert( flag );

src/veh_type.h

@@ -194,6 +194,8 @@ struct vpslot_terrain_transform {
     //Both only defined if(post_field)
     int post_field_intensity;
     time_duration post_field_age;
+
+    void deserialize( const JsonObject &jo );
 };
 
 struct vp_control_req {