Improvements to duplicate suppression, cafe stop search, and configurability.

When suppressing duplicate cafes and place names, look back a certain distance along
the route rather than just the previous point of interest.

When searching for cafes, use SearchIntersect() to return all entries in the
bounding rectangle, not just the nearest.

Remove (most) hard-coded constants and allow these to be overriden by options
to the NewGPXSummarizer() constructor.

pkg/cafes/common.go

@@ -10,7 +10,7 @@ import (
 )
 
 // Size (in metres) of the bounding box around a stop
-const stopRectangleSize = 500
+const stopRectangleSize = 50
 
 type RefreshmentStop struct {
 	Name     string