datParen: Factor out HDF5 code

Sometimes we want to get the parent group and getting the
HDF5 root group is acceptable.
datCopy can now copy the root locator

@pwdraper was trying to do

 hcopy mosaic mosaic.ndf_1

and that requires datCopy finds the parent of "mosaic" in order
to correctly call H5Ocopy. Previously used datParen which is
designed to object to that, so now calls dat1GetParentID which
can explicitly be told that "/" is okay.