Move the partition logic into the base risk mixin

[Andrea Cerisara]

The logic that splits the exposure sites into blocks (openquake.job.Job._partition) is used only by the risk subsystem, but it's done in the main Job class. Since we can now launch just hazard jobs, I propose to move it into the risk subsystem (openquake.risk.job.general.RiskJobMixin), so we can:

* avoid an unnecessary parsing when computing only hazard jobs
* when computing hazard + risk jobs, we can read the file only once (storing the assets + creating blocks)