Added an error message when neato doesn't work (when using "gg UDG -du|-dur ...") that is affected by --silent (aka. -s)

Debugged, renamed CommomGraph ClassicGraph, placed all the UDG utils from RandomGraph into a new module, enhanced "gg <command> -s" such that is has effect on warnings, removed "rand" function

Added comments, enhanced 'gg -h', changed some names, replaced an UDG tool function with another, and other minor things.

Added GraphGen to dune. The command is gg. Also added some comments to some .mli files, and reorganized a bit the functions.

Added the tool genGraph, to create .dot graph files (random ones or not, depending on the graph type).