Improve error if no matplotlib

[lauramurgatroyd]

If we don't have matplotlib we get an error like

To use colormaps other than:  ['viridis', 'plasma', 'inferno', 'magma'], please install matplotlib.
Traceback (most recent call last):
  File "C:\Users\lhe97136\Work\GitHub\CCPi-Users-Antikythera\Visualization\resample-ui.py", line 635, in <module>
    window = AntikitheraGUI()
  File "C:\Users\lhe97136\Work\GitHub\CCPi-Users-Antikythera\Visualization\resample-ui.py", line 91, in __init__
    self._plane_clipping()
  File "C:\Users\lhe97136\Work\GitHub\CCPi-Users-Antikythera\Visualization\resample-ui.py", line 199, in _plane_clipping
    observePlaneClipping(viewer1, viewer1.getInteractor(), "KeyPress")
  File "C:\Users\lhe97136\Work\GitHub\CCPi-Users-Antikythera\Visualization\plane_clip_slice.py", line 44, in observePlaneClipping
    transfer_function = CILColorMaps.get_color_transfer_function('gray',
  File "C:\Users\lhe97136\AppData\Local\miniconda3\envs\cilviewer\lib\site-packages\ccpi\viewer\utils\colormaps.py", line 460, in get_color_transfer_function
    colors = cls.get_color_map(cmap)
  File "C:\Users\lhe97136\AppData\Local\miniconda3\envs\cilviewer\lib\site-packages\ccpi\viewer\utils\colormaps.py", line 453, in get_color_map
    return colors
UnboundLocalError: local variable 'colors' referenced before assignment

The first line of the error (actually its just a print) is the useful part - the rest is not useful.

We should either make sure it returns nothing if matplotlib not found, or make it raise an exceptio rather than just doing a print.

CILViewer/Wrappers/Python/ccpi/viewer/utils/colormaps.py

Lines 432 to 451 in 65c5949

	class CILColorMaps(object):
	@staticmethod
	def get_color_map(cmap):
	if not cmap in _color_map_dict.keys():
	try:
	from matplotlib import cm
	colors = []
	for x in range(0, 255):
	color = cm.get_cmap(cmap)(x)
	colors.append([color[0], color[1], color[2]])
	except ImportError:
	print("To use colormaps other than: ",
	"{}, please install matplotlib.".format(str(list(_color_map_dict.keys()))))
	else:
	colors = _color_map_dict[cmap]
	return colors