Gsim validation

Companion to https://github.com/gem/oq-risklib/pull/100. The tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/910
Yes, .imtls is a property returning an OrderedDict (see https://github.com/gem/oq-risklib/blob/master/openquake/commonlib/oqvalidation.py#L145)