Behavior of gg_specieslevel_web_by_web when not having shared species among the webs

[valentinitnelav]

If you have multiple webs/networks and want to run gg_specieslevel_web_by_web in one go, but do not have shared species among the webs, then this will fail gracefully :)
In such cases, run boot_specieslevel() for each web separately. Make an example to clarify this behavior. Adapting the code do deal with this might be too time consuming and not a good investment right now.