defer (re)opening checkpoint files until we need to

[Roland Haas]

this patch defers opening checkpoint files until we need to do so because either we need to browse them or open a dataset for reading. This speeds up open_one_file_at_a_time since it will not open a hdf5 file that was already parsed and does not contain useful datasets.

Keyword: CarpetIOHDF5

[Erik Schnetter]

Please apply.

[Roland Haas]

Applied as carpet hash cfd1f43f4b7f "CarpetIOHDF5: defer opening data files until we have to to read a"