DAG visualisation

[aezarebski]

model objects in rev can output a DOT file.
the DOT package on CRAN provides functionality for writing this to a PS or SVG file.
could we please have some functionality in RevGadgets to stream line this process.

something like the following functions would be sufficient.

file_as_single_string <- function(fn) {
    readChar(fn, file.info(fn)$size)
}

dot2ps <- function(dot_file, ps_file) {
    dot_str <- file_as_single_string(dot_file)
    DOT::dot(dot_str, file=ps_file)
}

DOT does seem to require some stuff to be installed though which is a little unfortunate, but the error messages where very clear so that's nice.