Refactoring of risklib

The changes here are

1) now the risklib calculator for classical risk works in terms of poes, allowing a simplification in the hazard_getters on the engine side (see https://github.com/gem/oq-engine/pull/1615)
2) the workflow objects now depend on the `IMT` and `taxonomy` directly (needed to implement 1), but it is clearer and a good idea anyway, since it makes debugging of the risk calculators easier).
3) several questionable tests using only mocks have been removed, since I am going to add real tests
once the classical_risk calculator enters in master (the work is in progress).

The tests are green: https://ci.openquake.org/job/zdevel_oq-risklib/135