PPM Images

First pixel is at the top-left of the image, subsequent pixels are filled along a row.

1	write_pgm_r(image, filename)

`image`	Integer values preferably, but numeric values allowed. Length should be nrow * ncol, but it is up to the caller to check. Values are not checked for the presence of NAs.
`filename`	Filename to write to. Will be silently overwriten if it already exists.

If values is of type double, then they will be scaled to integers in the range [0,255]

For write_pgm() the caller is expected to have done all sanity checking as the input is not checked for correctness, i.e.

Not checked that is.numeric(values)
Not checked that length(values) = nrow * ncol
Not checked that all integer values in range [0, 255] - Values outside the range [0,255] will be written modulo 256
Not checked that there are no NA values