Fix #113 - enable implementations of GtfsTransformStrategy to use GTFS extensions

[kurtraschke]

Summary:

Fix #113 by adding methods (with empty default implementations) to GtfsTransformStrategy to permit transforms to register extensions (using onebusaway-csv-modules's extensions mechanism) which they will use during reading and/or writing.

Expected behavior:

Implementations of GtfsTransformStrategy can implement one or both of UpdateReadSchema and UpdateWriteSchema to register GTFS extension classes.

Note: There is no deconfliction performed: if multiple transforms are invoked in a single operation, and they register different extensions which use the same column names on the same objects, undefined behavior may result.

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[sdjacobs]

Note: There is no deconfliction performed: if multiple transforms are invoked in a single operation, and they register different extensions which use the same column names on the same objects, undefined behavior may result.

I'm noticing that it's also true that if multiple transforms register the same extension, bad behavior results. In particular, the column will be output twice:

stop_id,stop_name,stop_lat,stop_lon,location_type,parent_station,wheelchair_boarding,sample_extension,sample_extension

Personally, I'd prefer to address this issue, rather than leave it to users to decide which transform should declare extensions, if multiple transforms need the same extension. (I'd think this is a common usage pattern: agency X may make use of extension field E, which could be populated in a few different ways by different transforms.)

Something like the following could work in GTFSTransformStrategy:

public List<Pair<Class<? extends HasExtensions>, Class<?>> getDeclaredExtensions();

and then GTFSTransformer could add all extensions to a set before calling DefaultEntitySchemaFactory.addExtension. Thoughts?

[sheldonabrown]

Thanks @kurtraschke