hdf5_ep

(1) Add/remove comments.
(2) A temporary fix to address the test/objcopy.c: test_copy_group_deep() test failure with
the family driver.  The test failure occurs with these configurations in objcopy.c:
--with shared src messages (CONFIG_SHARE_SRC)
--without shared dst messages (CONFIG_SHARE_DST)
--with latest format for source file (CONFIG_SRC_NEW_FORMAT)
--without dense attributes (CONFIG_DENSE)
--with latest format for destination file (CONFIG_DST_NEW_FORMAT)
The temporary fix is in src/H5MFaggr.c (see comments above #ifdef REPLACE/#endif).