code: public Extensions getOrCreate(String tableName, String columnName,
  			String author, GeometryType geometryType) {
  
  		String extensionName = getExtensionName(author, geometryType);
  		String description = isGeoPackageExtension(geometryType) ? GEOMETRY_TYPES_EXTENSION_DEFINITION
  				: USER_GEOMETRY_TYPES_EXTENSION_DEFINITION;
  		Extensions extension = getOrCreate(extensionName, tableName,
  				columnName, description, ExtensionScopeType.READ_WRITE);
  
  		return extension;
  	}
docs: Get or create the extension, user defined geometry type
  
  @param tableName
  table name
  @param columnName
  column name
  @param author
  user defined author
  @param geometryType
  geometry type
  @return extension
  @since 1.1.8
  @deprecated as of 1.2.1, On August 15, 2016 the GeoPackage SWG voted to
  remove this extension from the standard due to
  interoperability concerns. (GeoPackage version 1.2)