added equality method for contexts objects
updated check_gsim script to check that context objects are not changed after calling get_mean_and_stddevs
fixed AB2003 and Zhao2006 and updated verification tables to reveal the bug
fix same error for two other GMPEs (AB1995 and Frankel1996)
merged latest master
Merge pull request #181 from monellid/avoid-changing-rup-context

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