Risk calculators refactoring

[matley]

With the current implementation of risk calculators we have a relevant issue.

It is not easy to implement unit tests. The main reason is because the "units" of the calculators assume that a complete environment has been setup (an hazard job/calculation and a risk job/calculation).

Mocking is not always a viable option (with this implementation) as you often need to mock all the ORM machinery.

Moreover, test execution is a kind of epic, because of the extra computation, as well as disk writing, needed to run a single "unit".

Furthermore, the current celery task accepts too many parameters, making difficult to test every combination of the inputs.

We need to refactor them, such that:

1) reading/writing from the db is separated in specific modules/functions
2) the new units can be tested with a tiny setup (involving few mock objects)
3) the new units have a better function signature (less input params, few output values)

[matley (matley)]

https://github.com/gem/oq-engine/pull/1164/