Fix loading of histogram data for slide scans

[adamltyson]

For some reason, slide scans fail when loading the histogram summaries. A fix to skip this step is added at

slidebook-python/sld/slidebook.py

Lines 18 to 24 in 7bf6906

	try:
	self.histogram_summary = np.load(
	self._histogram_summary_filename,
	mmap_mode=mmap_mode,
	)
	except ValueError:
	self.histogram_summary = None