WIP python bindings via pybind11

[apoorvalal]

create bindings in python/ that uses pybind11 to wrap grf code. Developing with regression_forest as first end-to-end example. Progress documented in python/notebooks/scratch.ipynb

[erikcs]

Very nice Apoorva, maybe you could convert this to a "Draft PR" then just use this as a playground for learning and fighting with C++/bindings using grf's build system at your own pace? I can happily give some input if you tally up questions. For a general scaffold I think anything James Yang does is awesome and looking for inspiration there is probably a good idea: https://github.com/JamesYang007/adelie

[apoorvalal]

Thanks Erik! Yeah, that was the plan (i was using the low-fi solution of prefixing with Work in progress but draft PR works better).

I also spent some time re-building skgrf from scratch in cython and found it works fine now, btw; I might develop that in parallel since it is less daunting to have to juggle serialization / pointers etc on cython [all the wrapper code lives in one place in contrast to the pybind11 solution).

Will keep you posted