v2.1.1. no support for members in ranges

[glenrobson]

If you have a range with members which is allowed in v2.1.1 it says:

WARNING: Setting non-standard field 'members' on resource of type 'sc:Range'

I think i need to add members to:

https://github.com/iiif-prezi/iiif-prezi/blob/e490571b370f7173c0f5cb14e0d8d2550d403d47/iiif_prezi/factory.py#L1445

there seems to be a list of 2.1 extra properties:

https://github.com/iiif-prezi/iiif-prezi/blob/e490571b370f7173c0f5cb14e0d8d2550d403d47/iiif_prezi/factory.py#L100