Add gcs() method to compositeGeometryFeature

All geometries must share the same GCS, so check this during
construction and save the value.
compsiteGeometryFeature should inherit from geoModule.feature
geoModule.planeFeature is subclass of geoModule.feature

Call geoModule.feature constructor rather than vglModule.actor
Add container member to layer

Store the container (often a map) a layer is associated with.
Add referenceLayer member

Used to indicate if the layer is the reference layer for the
container.
Mark map layer as the reference layer
Set GCS for openStreetMap layer

Set GCS to "EPSG:3857"
Add wordToGcs(...) method

Convert from world coordinates to the GCS of the reference layer
Guard against map layer in incorrect GCS

The GCS of the map layer must match that of the map
Add worldToDisplayGcs(...) to map

Allow the convertion form world coordinates to the display GCS