Revert "Merge branch 'master' of github.com:robintw/Py6S"

This reverts commit 90b478fbb445f3e7ebcf0a317edfa05a243d88ee, reversing
changes made to 287c70991998b9bf680c50f4dc030191db504d85.
Revert "Changed internal functions to start with _, thus hiding them from autocomplete"

This reverts commit 287c70991998b9bf680c50f4dc030191db504d85.
Revert "Added source to Landsat 8 SRFs"

This reverts commit 6cd305bf095e22523c0038e5b589f43ec9d51d5c.