Remove the Python API in favor of a dependency on lightkurve

[barentsen]

Now that PyKE v3.1.0 is released, let's work towards a v.3.2.0 release in which the new API (i.e. the LightCurve, KeplerTargetPixelFile, SFFCorrector, .... classes) is removed in favor of a dependency on the new lightkurve package.

This means that several modules can be removed from pyke, and slowly the command-line tools should evolve towards become the command-line interface for lightkurve functionaility.