Execute Goblint in incremental mode

[sim642]

After goblint/analyzer#358 it's possible to use Goblint's incremental analysis directly, so the plugin should do that using the incremental.load and incremental.save options.

Another possibility might be to not force GobPie to run Goblint with those options itself, but just require that the user enable them via #2 themselves. That would allow using the plugin both incrementally and non-incrementally without implementing anything. On the downside, if the user forgets to enable incremental in the conf file, all the analysis will be non-incremental by default, which will be silently slower than expected.