Possible workaround for truncation

[mmpagani]

[micheles]

The tests break because the scaling is a vector and not a scalar. A possible solution is to use a mean scaling factor:

                scaling = ((stddev_inter + stddev_inter) /
                           (stddev_inter**2 + stddev_intra**2)**0.5).mean()

[micheles]

Following the analysis in https://github.com/gem/oq-engine/pull/7151 this approach is not enough to solve the problem, since the issue is present also in the case where only STDDEV_TOTAL is defined, i.e. it is not caused by the effect that we are trying to address here.

[micheles]

Given the discussion of this morning, I would close this PR.

[micheles]

Reopened since Robin is working on this.