Introduce a GmfComputer class calling gsim.make_context only once

See https://bugs.launchpad.net/oq-hazardlib/+bug/1321998. The tests are green: https://ci.openquake.org/job/zdevel_oq-hazardlib/80/

There is a companion pull request on the engine: https://github.com/gem/oq-engine/pull/1441