Merging ultranest runs

[JohannesBuchner]

The points.hdf5 files should be made mergable.

With the merged points file, one can read it with read_file function or resume from it on a single CPU setting max_ncalls to 1, to post-process without computing further.

How to implement:

1. concatenate the 'points' tables inside (there is only one, have a look with h5ls)
2. sort it by Lmin.
3. write out the concatenated file.