Make sure that context objects are not changed inside 'get_mean_and_stddevs'

Fix https://bugs.launchpad.net/oq-hazardlib/+bug/1291016.

I extended the check_gsims.py script (which is used in all GMPE tests) to check that after calling 'get_mean_and_stddevs' the context objects  are not changed.

I fixed 4 GMPEs (Atkinson and Boore 1995/2003, Zhao et al. 2006 and Frankel 1996 - NSHMP 2008) which were changing the rupture context object. For two of them (AB 2003 and Zhao 2006) I had to update the tables to let the bug showing up

Test are running here: https://ci.openquake.org/job/zdevel_oq-hazardlib/119