Introduced a GsimLogicTree class
Fixed the random seed
Improvements to GsimLogicTree
Simplified the GsimLogicTree API
Turned logic tree instances into iterable yielding realizations
Improved docstring and comments
Some renaming to make the logic tree management more general
Merge branch 'master' of github.com:gem/oq-commonlib into fix-gmf-duplication-2
Removed unused imports
Added more validation of the logic tree
Special case when there are no tectonic region types
Merge pull request #9 from gem/fix-gmf-duplication-2

Fix gmf duplication