Make forward/reverse weight/offset calculated at query time,

rather than being cached in the StaticRTree.  This means we
can freely apply traffic data and not have stale values lying
around.  It reduces the size of the RTree on disk, at the expense
of some additional data in RAM.

features/step_definitions/data.rb

@@ -10,6 +10,10 @@ Given /^the extract extra arguments "(.*?)"$/ do |args|
     set_extract_args args
 end
 
+Given /^the prepare extra arguments "(.*?)"$/ do |args|
+    set_prepare_args args
+end
+
 Given /^a grid size of (\d+) meters$/ do |meters|
   set_grid_size meters
 end
@@ -146,6 +150,12 @@ Given /^the raster source$/ do |data|
   end
 end
 
+Given /^the speed file$/ do |data|
+  Dir.chdir TEST_FOLDER do
+    File.open("speeds.csv", "w") {|f| f.write(data)}
+  end
+end
+
 Given /^the data has been saved to disk$/ do
   begin
     write_input_data