Added Ellipsoidal SRTM dataset

[kxtells]

This PR adds new product SRTM1_ELLIP with the opentopography datasource containing height from ellipsoid which is useful for some kind of analysis.

Also added a test for the new function srtm_ellip_tiles_names, ran with tox and python3.7 with no errors in sight. But did not test with the rest of interpreters:

 summary 
  docs: commands succeeded
  py27: commands succeeded
ERROR:  py34: InterpreterNotFound: python3.4
ERROR:  py35: InterpreterNotFound: python3.5
ERROR:  py36: InterpreterNotFound: python3.6
ERROR:  pypy: InterpreterNotFound: pypy
  qc: commands succeeded

Thanks for the elevation library, it has been very useful to me in the past so I'm more than happy to contribute :-D

[alexamici]

Thanks a lot!

[kxtells]

Thank you for your time :-)