Quantile hazard calculation fails when multiple logic tree branches are enumerated

[Lars Butler]

Applies to classical hazard quantile post-processing.

In the case of end-branch enumeration with more than 1 branch, numpy.interp (used in quantile_curves_weighted) cannot handle the weights input properly. `weights` is passed as a list of decimal.Decimal types.

Numpy throws back this error: "TypeError: array cannot be safely cast to required type". This doesn't appear to be a problem when there is only a single end branch in the logic tree (and so the single weight is 1.0).

[Lars Butler (lars-butler)]

Patch: https://github.com/gem/oq-engine/pull/879