engine:

Refactored `get_input` and renamed it `get_or_create_input`. This can
now be used the same way as before, but can also be used for
bootstrapping calculations, in contexts outside of the engine as a
command line tool.
calcs/hazard/general:

Stop using the `engine.get_input` API function, and instead use the
simplified `engine.get_or_create_input` function.
engine:

Fixed some logic in an assertion.
engine:

Updated parse_config to have less side-effects, and simplified its
return interface.

Instead of creating Input records, we just return a dict of the input
model file paths and leave the Input creation to a later function in
the workflow.
engine:

Updated hazard and risk calc creation functions to match the new
interface for parse_config.
tests:

Made all tests pass with the changes to engine.parse_config.
Merge pull request #1240 from larsbutler/better-input-loading

Better input loading (update to `engine` API)

[r=matley] [f=*1206913]
merge commit