NHLIB: allow gmf calculation with GMPEs that provide only total standard deviation

[Damiano Monelli]

Currently, the ground motion field calculator (https://github.com/gem/nhlib/blob/master/nhlib/calc/gmf.py#L26) requires GSIMs to provide both the inter and intra event standard deviations (see https://github.com/gem/nhlib/blob/master/nhlib/calc/gmf.py#L99).

However some GMPEs or IPEs may provide only total standard deviation (see https://github.com/gem/nhlib/blob/master/nhlib/gsim/sadigh_1997.py#L52).

We need to modify the workflow within the GMF calculator in order to cope with this situation.

So if a GSIM provides only total standard deviation then, the correlation model must be None (because it's not used) and we need to compute only mean and total standard deviation at the sites of interest, that is:

        mean, [stddev_total] = gsim.get_mean_and_stddevs(
            sctx, rctx, dctx, imt, [StdDev.TOTAL]
        )

we can then compute the total residual:

        total_residual = stddev_total * distribution.rvs(size=(len(sites),
                                                               realizations))

and then compute the gmf as:

        gmf = gsim.to_imt_unit_values(mean + total_residual)

In case of a gsim that provides both inter and intra event standard deviations the workflow remains unchanged.

[Lars Butler (lars-butler)]

Patch: https://github.com/gem/nhlib/pull/72