Fix imshow call in edtools.find_rotation_axis

[dagewa]

edtools.find_rotation_axis XDS.INP crashed for me with

Traceback (most recent call last):
  File "/home/fcx32934/sw/cctbx/conda_base/bin/edtools.find_rotation_axis", line 8, in <module>
    sys.exit(main())
  File "/home/fcx32934/sw/cctbx/conda_base/lib/python3.8/site-packages/edtools/find_rotation_axis.py", line 362, in main
    plot_histo(H, xedges, yedges, title=f"omega={omega_final:.2f}$^\circ$ | var={var:.2f}")
  File "/home/fcx32934/sw/cctbx/conda_base/lib/python3.8/site-packages/edtools/find_rotation_axis.py", line 81, in plot_histo
    plt.imshow(H.T, interpolation='nearest', origin='low',
  File "/home/fcx32934/sw/cctbx/conda_base/lib/python3.8/site-packages/matplotlib/pyplot.py", line 2724, in imshow
    __ret = gca().imshow(
  File "/home/fcx32934/sw/cctbx/conda_base/lib/python3.8/site-packages/matplotlib/__init__.py", line 1447, in inner
    return func(ax, *map(sanitize_sequence, args), **kwargs)
  File "/home/fcx32934/sw/cctbx/conda_base/lib/python3.8/site-packages/matplotlib/axes/_axes.py", line 5519, in imshow
    im = mimage.AxesImage(self, cmap, norm, interpolation, origin, extent,
  File "/home/fcx32934/sw/cctbx/conda_base/lib/python3.8/site-packages/matplotlib/image.py", line 902, in __init__
    super().__init__(
  File "/home/fcx32934/sw/cctbx/conda_base/lib/python3.8/site-packages/matplotlib/image.py", line 246, in __init__
    cbook._check_in_list(["upper", "lower"], origin=origin)
  File "/home/fcx32934/sw/cctbx/conda_base/lib/python3.8/site-packages/matplotlib/cbook/__init__.py", line 2266, in _check_in_list
    raise ValueError(
ValueError: 'low' is not a valid value for origin; supported values are 'upper', 'lower'

This PR fixes that for the version of matplotlib I am using (3.3.4). I checked the docs for the minimum version for edtools (3.2.1), and the form origin="lower" appears to be fine there too.

[Taimin]

It's looking great! Thanks a lot, David.