Open external links as read-only when combining files

[paulmueller]

The combine_h5files method should open the external files in read-only mode. This is now possible with h5py 3.9.0.

• https://github.com/DC-analysis/dclab/blob/master/dclab/rtdc_dataset/linker.py
• https://api.h5py.org/h5p.html#h5py.h5p.PropLAID
• https://api.h5py.org/h5p.html#h5py.h5p.PropLAID.get_elink_acc_flags
• https://forum.hdfgroup.org/t/can-we-use-external-links-to-read-only-data-in-writable-files/9390
• Expose low level getter/setter for elink access flags and fixes missing lapl arguments h5py/h5py#2244
• https://docs.h5py.org/en/stable/whatsnew/3.9.html#exposing-hdf5-functions