Rdump format

[ahartikainen]

Hi, quick question if we had some restrictions for the Rdump format?

For example, R dataframes cannot be read with PyStan functions.

https://github.com/stan-dev/example-models/blob/master/misc/gaussian-process/gp-fit-ARD.data.R

If they are "valid", then we need to update PyStan read function.