Event-Based Hazard Spatial Correlation QA tests

Addresses https://bugs.launchpad.net/openquake/+bug/1099467.

This branch primarily introduces 3 more test cases for the EB hazard calculator which test various spatial correlation scenarios.

There are also some small updates to `db.fields` and `db.models` modules:

1) I fixed a minor bug in `truncation_level` field to allow for blank values.
2) I added functionality to the `GmfSet.__iter__` so that we can easily retrieve all ground motion fields for a given site.