Added some documentation for HazardCurveDBReader.
Added some documentation for HazardCurveDBReader.


Former-commit-id: 20aff31a7cbb7c2688c895f17f73742ff581415e [formerly 44c623e041063fa407d633b1202001765c68707c]
Former-commit-id: 6f54ce057fc73624ac14d056fff427c60e8d4e31
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine