File handling

Pingwyn has the following functions for file handling

pingwyn.list_variables(obj, dtype)
pingwyn.load_variable(obj, dtype, name)
pingwyn.open(path, *args, **kwargs)	Open a data file.
pingwyn.open_all(path[, force_glob, ...])	Opens multiple data files.
pingwyn.save(path)	This function is not implemented
pingwyn.save_variable(obj, dtype, name[, ...])