Added a class to extract the closest object

The tests are green: https://ci.openquake.org/job/zdevel_oq-hazardlib/99
I think here you can use the method `_geodetic_min_distance(self, mesh, indices=True)` of `openquake.hazardlib.geo.mesh`.

In the constructor build a `Mesh` with the given longitude and latitude values. The above method will give you the index of the element closest to the `lon` and `lat` values passed in the method (which need to be put in a mesh object too). With that index you can extract the closest object.