input/logictree:

Corrected a comment.
input/logictree:

Reduced the coupling between logictree and oq-hazardlib by using the `type`
attrs of source and mfd objects to check the type, instead of `isinstance`
checks.
input/logictree:

The map of GSIMs is now passed as a dependancy to GMPELogicTree.

This helps to decouple oq-hazardlib from the logic tree code.
tests/input/logictree_test:

Added a characteristic fault source to the `test_source_type` test case.
tests/input/logictree_test:

Pass the GSIM map to the super constructor in _TesteableGMPELogicTree.
tests/inputs/logictree_test:

Removed an unnecessary test.