Issue cleanup

[khelm]

This PR:
Done - 15) Should we create the more generic "Protocol" as a Performed Plan and then tag using nidm:AcquisitionModality? Answer = yes.

3. score should go under mathematical_entity

4. what is a scale_item?
   There are all types of scales in SIO - decimal scale, integer scale, etc. Do we need any of these?
   The issue here is that both can be either a sio:'language entity' or a sio:'mathematical entity', so I have
   to make the parent more general than each (except now onl:scale_item and onl:score are on the same level as both of those entities which seems not quite right since score and scale_item has an "is a" relationship with those entities. This will have to be revisited later as a separate issue.

Done 2) The definition of onl:scale is consistent with it being a sio:"language entity"

Done 6) nidm:SessionObject - I think this should be a collection. That way each scan file can be a member of the collection, but you can still retrieve the whole object if you want all of the scans from a given session. This is how it's defined by Nolan and "hasPart some AcquisitionObject".
Put nidm:SessionObject under prov:collection and get rid of the placement of prov:collection under sio:set. Remove sio:set as we don't really need it since we have prov:Collection.

Done 2) nidm:Model could go under sio:'language entity' -> sio:'description'