Batch processing: add a progress indicator

[GreatEmerald]

At the moment the batch functions are silent, so it is hard to gauge how long the processing is really going to take. It would be nice if there was a progress indicator.

raster includes a progress indicator for a bunch of functions by passing progress="text" to writeRaster(). However, that is not very useful when tasks are running in parallel, as the indicators overlap one another. An alternative would be to show the number of tasks complete vs the number of tasks remaining, but for a small number of tasks (cores > tasks) that would not be that useful either.

Maybe there's a way to print a whole bunch of progress indicators one over the other, though?

[loicdtx]

The pbmcapply package would solve that issue. Not sure a progress bar justifies the extra dependency though :-) I'll investigate that next time I get to work on the package.