extend SiteContext to accept also sites coordinates (lons and lats)

In this pr I'm extending the site context object so that it can contains the geographical coordinates (longitude and latitudes) of the sites for which the parameters are given (https://bugs.launchpad.net/oq-hazardlib/+bug/1355716).

This is needed for site corrections in the Japanese national seismic hazard model which take into account distance of a site with respect to volcanic axis and subduction trench axis