input/source:

Added `area_source_to_point_sources` converter function.
tests/input/source_test:

Added tests for `area_source_to_point_sources`.
input/source:

Added `optimize_source_model` function.
bin/openquake:

Added `--optimize-source-model` option.
tests/input/source_test:

Added a test to exercise `optimize_source_model`.