Point source block size

Part of https://bugs.launchpad.net/openquake/+bug/1096867.

This branch introduces a `point_source_block_size` parameter, so that point sources can be grouped in larger chunks. We've found that it's optimal to distribute 1 area, simple, complex, or characteristic fault source per task. For point sources, it's optimal to distribute several hundred per task, since the task overhead outweighs the computation time for a single point source dramatically.

There are also several cleanups here:

- Deleted some obsolete and slow test code
- Updated the parameters for a very slow test to speed it up dramatically; see `tests/input/source_test.py`
- Renamed `*CalculatorNext` classes to `*Calculator`