Support arbitrary distance metrics

[motiwari]

We should have an easy way for users to incorporate their own dissimilarity metrics. Users should be able to write their dissimilarity metric in Python, which is then called via the C++ code.

User feedback suggests allowing users to pass the reference to the function that will compute distances. Asking the user to supply the O(n^2) distance matrix is of course infeasible.

This is a several-step task, including:

1. Making the Python function callable via the C++ (C++ bindings for the Python code); perhaps via some translation
2. Updating the C++ code to use this custom function. Some relevant code

[tanweer-mahdi]

Hi @motiwari , do you currently recommend implementing custom distance metrics locally? Or shall I create a branch, code the distance metric in C++ as you suggested in the README and create a PR?

[motiwari]

@tanweer-mahdi the latter option is welcome and I'd be happy to review a PR!