Get geotiff working from tunnelx documented for windows users

[goatchurchprime]

Up to date version of tunnel is at tunnel2018.jar at https://bitbucket.org/goatchurch/tunnelx/downloads/
This should generate a geotiff every time you make a png file (from the Print tab). You can tell you've got this version if you see "EPSG:32630" in a text area on the print tab.

When a png file is created (and the coordinates are correct in the centreline, because it's been derived from all.svx instead of just a subsketch with no *fix involved) it calls the program gdal_translate on it.

We need this command to be able to run on windows machines, or it will never work. Also explain how to install it on linux and mac machines reliably. (I could put the answers, when we find them, into the error message from tunnel when it fails.)

[goatchurchprime]

This also includes getting the survex engine to run from the command line as well to generate the centreline used by tunnel