Autoplotter doesn't like single-point data sets

[james-trayford]

...as far as I can tell.

Currently circumventing by plotting the same point twice. Example traceback for a plot where the data set is a single point

Unable to create plot h2_frac_func_stellar_mass due to exception: Illegal slicing argument for scalar dataspace.
Traceback:
  File "/cosma/home/dp004/wmfw23/.local/lib/python3.6/site-packages/velociraptor/autoplotter/objects.py", line 1028, in create_plots
    file_extension=file_extension,
  File "/cosma/home/dp004/wmfw23/.local/lib/python3.6/site-packages/velociraptor/autoplotter/objects.py", line 864, in make_plot
    redshift_bracket=observational_data_redshift_bracket,
  File "/cosma/home/dp004/wmfw23/.local/lib/python3.6/site-packages/velociraptor/observations/__init__.py", line 98, in load_observations
    data.load(filename)
  File "/cosma/home/dp004/wmfw23/.local/lib/python3.6/site-packages/velociraptor/observations/objects.py", line 284, in load
    filename, dataset_name=f"{prefix}values", group_name="x"
  File "/cosma/home/dp004/wmfw23/.local/lib/python3.6/site-packages/unyt-2.7.1-py3.6.egg/unyt/array.py", line 1408, in from_hdf5
    data = dataset[:]
  File "h5py/_objects.pyx", line 54, in h5py._objects.with_phil.wrapper
  File "h5py/_objects.pyx", line 55, in h5py._objects.with_phil.wrapper
  File "/cosma/local/Python/3.6.5/lib/python3.6/site-packages/h5py-2.10.0-py3.6-linux-x86_64.egg/h5py/_hl/dataset.py", line 540, in __getitem__
    selection = sel2.select_read(fspace, args)
  File "/cosma/local/Python/3.6.5/lib/python3.6/site-packages/h5py-2.10.0-py3.6-linux-x86_64.egg/h5py/_hl/selections2.py", line 103, in select_read
    return ScalarReadSelection(fspace, args)
  File "/cosma/local/Python/3.6.5/lib/python3.6/site-packages/h5py-2.10.0-py3.6-linux-x86_64.egg/h5py/_hl/selections2.py", line 88, in __init__
    raise ValueError("Illegal slicing argument for scalar dataspace")

presumably this can be fixed with an explicit array declaration in dataset.py