mtk-gml: Use a separate default conversion function.
mtk-gml: Remove duplicates from mtk_features.
mtk-gml: Remove classes which are no longer updated in MTK.
mtk-gml: Add tags to some generic classes, just to be sure.
mtk-gml: Add tags to unclassified wreck.
mtk-gml: Change natural:bedrock to natural:bare_rock per OSM wiki.
mtk-gml: Add tags to Ilmarata (aerialway:unknown).
mtk-gml: Add names and other extra tags to highways.
mtk-gml: Add filterLayer function to skip some layers.
mtk-gml: Add some 'unclassified' feature classes.
mtk-gml: mtk_surveypoint(), tags and height to surveypoints.
mkt-gml: Tag unknown airfields with aeroway=aerodrome.
mtk-gml: Tag bell towers and make tower height text points towers.

Also do not tag chimney height text points.
mtk-gml: Add 'kuntatunnus' as ref for municipal boundary.
mtk-gml: Tag local address points with addr:full.
mtk-gml: Tag luonnosuojelualue with boundary=protected_area, protect_class=4.
mtk-gml: Tags for various text points for dwellings.

'Kaupungin nimi' -> place=town
'Muun kunnan nimi' -> place=village
'Kylän, kaupunginosan tai kulmakunnan nimi' -> place=suburb
'Talon nimi' -> place=isolated_dwelling
'Muu asutusnimi' -> place=isolated_dwelling
mtk-gml: Tag island text points with place=islet, instead of place=island.
mtk-gml: Do not call ustr() twice.
mtk-gml: Add tags to most 'nimi' (name) text points.