Add an R API using data.frame

[fritzo]

It would be great to provide a common interface in Python and R using pandas.DataFrame and data.frame, respectively.

Options:

• Wrap pytreecat in reticulate (as done by rstudio/tensorflow). Use either feather or upcoming dataframe conversion Pandas DataFrame and R data.frame translation rstudio/reticulate#2
• Reimplement the core of treecat in C++ and wrap in Rcpp