input/logictree:

Refactored logic tree implementation a bit to support reading of logic
trees from the DB. Also added `read_logic_trees_from_db`.
engine:

When bootstrapping a hazard calculation, parse the source model logic
tree and collect all of the source model file paths. Then load them
into the DB.
input/logictree:

Removed an obsolete function `read_logic_trees_from_fs`.
input/logictree:

Removed some obsolete _open_file functions.
tests/input/logictree_test:

Big test overhaul. Refactored some tests so that they pass, and removed
some obsolete ones.
calcs/hazard/general:

Use the new `read_logic_trees_from_db` function in the hazard
pre_execute.
tests/db/models_test:

Updated a test per new input loading functionality.