Fix ZeroDivisionError crash in dials.compute_delta_cchalf

[jbeilstenedmands]

When running the program on any single dataset, this results in crash and Traceback:

  File "/dials/build/../modules/dials/command_line/compute_delta_cchalf.py", line 139, in <module>
    run()
  File "/dials/conda_base/python.app/Contents/lib/python3.8/contextlib.py", line 75, in inner
    return func(*args, **kwds)
  File "/dials/build/../modules/dials/command_line/compute_delta_cchalf.py", line 134, in run
    script.run()
  File "/dials/modules/dials/algorithms/statistics/cc_half_algorithm.py", line 220, in run
    self.algorithm.run()
  File "/dials/modules/dials/algorithms/statistics/cc_half_algorithm.py", line 496, in run
    statistics.run()
  File "/dials/modules/dials/algorithms/statistics/delta_cchalf.py", line 259, in run
    self._cchalf = self._compute_cchalf_excluding_each_group()
  File "/dials/modules/dials/algorithms/statistics/delta_cchalf.py", line 300, in _compute_cchalf_excluding_each_group
    cchalf = compute_cchalf_from_reflection_sums(
  File "/dials/modules/dials/algorithms/statistics/delta_cchalf.py", line 152, in compute_cchalf_from_reflection_sums
    return compute_mean_cchalf_in_bins(bin_data)
  File "/dials/modules/dials/algorithms/statistics/delta_cchalf.py", line 127, in compute_mean_cchalf_in_bins
    mean_cchalf /= count
ZeroDivisionError: Please report this error to dials-support@lists.sourceforge.net: division by zero

This should not be reachable with proper input checking, so raise an error in initialisation if only one dataset/group is given.

[codecov]

Codecov Report

Merging #1892 (ae0dbd1) into main (2751e2f) will increase coverage by 0.00%.
The diff coverage is 95.55%.

❗ Current head ae0dbd1 differs from pull request most recent head 170fa62. Consider uploading reports for the commit 170fa62 to get more accurate results

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  Coverage   67.20%   67.21%           
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  Files         619      619           
  Lines       70014    70036   +22     
  Branches     9720     9722    +2     
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+ Hits        47054    47075   +21     
- Misses      20993    20995    +2     
+ Partials     1967     1966    -1