export/hazard:

Fixed the retrieval of `hazard_curve_multi` metadata. IML/IMT
information wasn't being collected from the correct `Output`.
tests/export/hazard_test:

Do XML validation for `hazard_curve_multi` outputs as well.

Previously, all artifacts except for this type were being validated.
Merge pull request #1224 from larsbutler/fix-multihazcurve-export

Fix `hazard_curve_multi` export

[r=micheles] [f=*trivial]