def urls(self, plugin_name, easy_instance_field):
        return None
Combine layers from two roipacks. Layers / svg file from first is maintained.
dataset/ETHPy150Open Alir3z4/django-databrowse/django_databrowse/sites.py/DatabrowsePlugin.urls--crossover with--dataset/ETHPy150Open gallantlab/pycortex/cortex/svgroi.py/ROIpack.__add__