Assign (some) offline navdata to a survey.

There is no concept of ‘current survey’ in Dougal, and
assigning navigation data to a particular survey is full
of edge cases but sometimes it is necessary or at least
convenient to do so.

This commit implements once such strategy, which consists
of checking the distance to the preplots of all active
surveys (well, those that do have preplots anyway) and
picking the nearest one.

To reduce load, we only do this every once in a while as
governed by the `offline_survey_detect_interval` option
in the configuration.

This strategy is only active if the configuration option
`offline_survey_heuristics == "nearest_preplot"` for the
corresponding navigation header.

etc/config.yaml

@@ -21,6 +21,10 @@ navigation:
                 # Anything here gets passed as options to the packet
                 # saving routine.
                 epsg: 23031 # Assume this CRS for unqualified E/N data
+                # Heuristics to apply to detect survey when offline
+                offline_survey_heuristics: "nearest_preplot"
+                # Apply the heuristics at most once every…
+                offline_survey_detect_interval: 10000 # ms
 
 
 imports: