BUGFIX, features

• fix bug where finalize() closed ALL files instead of just one file.
• add hdf5_close() procedure, good to close at end of program to ensure no dangling files remain
• add flush() method, useful for long-running programs where periodic flushes help preserve data thus far if program crashes (run out of HPC time allotment for example)
• add is_open property, mostly used internally to guard against users calling methods on closed files, or double-initializing an open handle.