nhlib: Disaggregation calculation crashes when a source produces no ruptures

[Lars Butler]

I encountered this crash while trying to use the nhlib disagg calculator in OQ Engine. This is the error I get:

 ValueError('zero-size array to minimum.reduce without identity',)
Traceback (most recent call last):
  File "/Users/larsbutler/proj/oq-engine/openquake/utils/tasks.py", line 212, in wrapped
    task_func(*args, **kwargs)
  File "/Users/larsbutler/proj/oq-engine/openquake/utils/stats.py", line 288, in wrapper
    result = func(*args, **kwargs)
  File "/Users/larsbutler/proj/oq-engine/openquake/calculators/hazard/disagg/core.py", line 65, in disagg_task
    compute_disagg(job_id, block, lt_rlz_id)
  File "/Users/larsbutler/proj/oq-engine/openquake/calculators/hazard/disagg/core.py", line 167, in compute_disagg
    **calc_kwargs)
  File "/Users/larsbutler/.virtualenvs/openquake/lib/python2.7/site-packages/nhlib/calc/disagg.py", line 109, in disaggregation
    coord_bin_width, truncation_level, n_epsilons)
  File "/Users/larsbutler/.virtualenvs/openquake/lib/python2.7/site-packages/nhlib/calc/disagg.py", line 205, in _define_bins
    int(numpy.floor(mags.min() / mag_bin_width)),
ValueError: zero-size array to minimum.reduce without identity

Doing some debugging, I discovered that this only occurs when a source produces no ruptures.

[Lars Butler (lars-butler)]

Also: This is only occurs when I use a rupture-to-site distance filter. Removing the filter prevents this error from happening.

[Lars Butler (lars-butler)]

After a discussion with Dr. Monelli, we have determined that the best way to handle this is

1) Check for empty bins data.
2) If they are empty, throw a runtime warning and return None from the disagg calculator. The warning should explain that no ruptures contributed to the hazard level at this site.

[Lars Butler (lars-butler)]

Patch: https://github.com/gem/nhlib/pull/73