Make sure that context objects are not changed by calling 'get_mean_and_stddevs'

[Damiano Monelli]

We need to make sure that context objects (https://github.com/gem/oq-hazardlib/blob/master/openquake/hazardlib/gsim/base.py#L548) are never changed by calling the method get_mean_and_stddevs (https://github.com/gem/oq-hazardlib/blob/master/openquake/hazardlib/gsim/base.py#L118) because this can change the state of the object without the user being aware of it.

For instance the AtkinsonBoore2003SInterNSHMP2008 GMPE is considered, the following happens:

In [1]: from openquake.hazardlib.gsim.atkinson_boore_2003 import AtkinsonBoore2003SInterNSHMP2008
In [2]: from openquake.hazardlib.gsim.base import RuptureContext, SitesContext, DistancesContext
In [3]: from openquake.hazardlib.imt import PGA
In [4]: from openquake.hazardlib.const import StdDev

In [6]: sites = SitesContext()
In [7]: sites.vs30 = array([760.])

In [8]: rup = RuptureContext()
In [9]: rup.mag = 6
In [10]: rup.hypo_depth = 50

In [11]: dists = DistancesContext()
In [12]: dists.rrup = array([40.])

In [13]: gmpe = AtkinsonBoore2003SInterNSHMP2008()

In [14]: gmpe.get_mean_and_stddevs(sites, rup, dists, PGA(), [StdDev.TOTAL])
Out[14]: (array([-3.80923033]), [array([ 0.52959457])])

In [15]: rup.hypo_depth
Out[15]: 20.0

As it can be seen, `rup.hypo_depth` is initially set to 50 but after calling `gmpe.get_mean_and_stddevs` the value is set to 20. This because the GMPE internally changes its value (https://github.com/gem/oq-hazardlib/blob/master/openquake/hazardlib/gsim/atkinson_boore_2003.py#L346)

[Michele Simionato (michele-simionato)]

https://github.com/gem/oq-hazardlib/pull/181