rhmapfile is in desperate need of documentation

[Jeremiah Rounds]

Hi,

I was trying to figure out how to do an old rhgetkey, and failing badly at it.

I tracked down the ~datadr~ source of https://github.com/tesseradata/datadr/blob/master/R/ddo_ddf_kvHDFS.R#L139    Once I did have the hint I ended up at: http://tessera.io/docs-RHIPE/functionref.html#rhmapfile, and finally https://github.com/tesseradata/RHIPE/blob/master/src/main/R/mapfile.functions.R

As far as I can tell there is no documentation on the Mapfile class other than the existence of its constructor? I think the most glaring omission is documenting the original "query-able key/value store" functionality Saptarshi laid out during his dissertation years.

Just thought I would note a sticking point =)