rename coastline, landmass and sea layers

Created by: das-g

From an email from @sfkeller:

Zusätzliche Tabellen: [...]

• Coastline_l, Landmass_a, Ocean_a: Drei neue Tabellen werden integriert und in den Auszügen mitgeliefert

These names for the new layers from geometalab/osmaxx-conversion-service#99 (based on http://openstreetmapdata.com/data/coast data) are more consistent with the other layer names than the current ones. As the Water polygons data contains

[p]olygons for oceans**, seas and large bodies of inland water**

(emphasis mine) i.e., not only oceans, the names should probably be coastline_l, landmass_a, sea_a or coastline_l, landmass_a, water_a.