validate_site_model bug fix, enabling a previously disabled qa test
updated test_validate_site_model to consider a degenerate case
Merge branch 'master' of https://github.com/gem/oq-engine into site_model_validation_bug_fix
removed old comment and unused import
Refactoring, rename of a constant, added a comment
Relaxed constraint db side, kept and update validation rule in validation.py
Added comment to clarify site model convex hull area expansion
export/hazard:

Use `iterator()` when fetching (potentially) large hazard curves, to avoid
caching them. This drastically reduces spikes in memory usage for the export.
export/hazard:

Slightly faster and slightly leaner (less memory usage) query for hazard
curve export.
export/hazard:

Updated HazardCurveData query to avoid returning and constructing millions of
Point geometry objects.
export/hazard:

When exporting hazard curves, order DB query results by `id`. This ensures
that results have a stable and consistent order.
replaced because with if :)
Merge pull request #1009 from gvallarelli/site_model_validation_bug_fix

Site model validation bug fix
Updated error message, now it's more helpful to the end user
Merge pull request #1010 from gvallarelli/relax_constraint_truncation_level

Relaxed constraint truncation level.
Merge pull request #1008 from larsbutler/hc-export-optimize

Optimized hazard curve export

[r=matley] [f=1099763]