It can now be either a string (file name) or an HDS.  I also enforce
the window size to be an integer value > 0.

It is clear that reading of the holography observation in the test
data directory is supposed to be tested.  Hence just read it and
make sure that the content has been read correctly.

SW-43: fix logging setup and adding the averaging selection the time window instead of the number of samples

SW-43: fix avoid modification of the source dict while storing. Fixing iteration over the attribute and the values of the h5 dataset.

SW-43: renamed function process_holography_dataset to a more specific name preaverage_holography_dataset