Expose the R hclust() members functionality in the Python interface

[dennisverspuij]

Hello Daniel,

Hereby my first draft to expose the members functionality in the Python interface. I have a few issues:

• I am not sure how to match the dtype of the members numpy.array array with the t_index C++ type, NPY_LONG works on my 64 bit system
• linkage_vector() yields different results than linkage() when using identical method and metric, and a members array with some values different than 1. It seems an implementation difference rather than a bug in my code, but I may be completely wrong

[dennisverspuij]

Hello Daniel, any chance on reviewing this?