Added some utilities which are useful when implementing hazard calculators in commonlib

I am adding a function `calc_hazard_curves` which is basically an alias for `hazard_curves` and three properties `x`, `y`, `z` to the Point object for compatibility with the export API. The tests are running here: https://ci.openquake.org/job/zdevel_oq-hazardlib/164/

PS: after discussion with Graeme, we are deprecating `hazard_curves` in favor of  `calc_hazard_curves` 
self.imts would be enough