Use correct ECEF coordinates

[rhodes73]

• Fix bug in geodesic to ECEF coordinate conversion function
• Re-enable use of ECEF coordinates in KD-Tree

As pointed out in #10, when using ECEF coordinates without this fix for the latitude in the z-coordinate, some lookups result in wrong results:

In [1]: import reverse_geocoder as rg
   ...: rg.search([(37.78674, -122.39222)])
Loading formatted geocoded file...
Out[1]:
[OrderedDict([('lat', '37.77993'),
              ('lon', '-121.97802'),
              ('name', 'San Ramon'),
              ('admin1', 'California'),
              ('admin2', 'Contra Costa County'),
              ('cc', 'US')])]

After the fix, this does not happen anymore:

In [1]: import reverse_geocoder as rg
   ...: rg.search([(37.78674, -122.39222)])
Loading formatted geocoded file...
Out[1]:
[OrderedDict([('lat', '37.77493'),
              ('lon', '-122.41942'),
              ('name', 'San Francisco'),
              ('admin1', 'California'),
              ('admin2', 'San Francisco County'),
              ('cc', 'US')])]

[nttams]

Hi,
I'm also looking for a solution to use ECEF in KDTree. This PR looks ok, why has not it been merged yet?
Thanks.

[Dobatymo]

This PR might finally fix this library, after being broken for most of the time.