hdf5_ep

[svn-r26476] Add support for I/O in all cases with fixed size datasets and no point
selections.
Add source dataspace extent patching (should allow closing the source file).
Note fill values and various other features are not yet supported.
Note there are still some code coverage assertions in the selection matching
algorithm - if you hit these try taking them out.
Note make check still fails in h5dump test (unrelated to this checkin).

Tested: ummon