Figure out how gvanno can fit in to the workflow

[oskarvid]

gvanno is a workflow that does genome variant annotation and would be very useful as an addition to Selma. Exactly how it could be added has not been decided, one suggestion is to allow it as an option when Selma is started. Another suggestion is to recommend it to users in the documentation or as part of the end-of-execution message that is shown when the workflow is finished successfully. Feel free to make other suggestions.