Need a pure python implementation

[rsdoiel]

Now that Python 3 is now widely used and supported (2 was very much alive and kicking when py_dataset was implemented relying on a C shared library approach). I think it makes sense to re-implement py_dataset as a pure Python program and skip the hard to maintain libdataset (Go C-share libraries aren't nearly as convenient to build and distribute as pure Go modules).

dataset is proving increasingly stable and now support three storage back ends in addition to pairtrees. It should be possible to port the functionality of the Go cli as python packages that then can replace the libdataset sub-module.