code: protected boolean has(String extensionName, String tableName,
  			String columnName) {
  		Extensions extension = get(extensionName, tableName, columnName);
  		return extension != null;
  	}
docs: Determine if the GeoPackage has the extension
  
  @param extensionName
  extension name
  @param tableName
  table name
  @param columnName
  column name
  @return true if has extension