Allow accessibleAt having more than one value
Currently, the field accessibleAt only allows one url. It could be, that there are more than one urls to a tool (imagine installations on different servers) but maybe this is more often the case for training-material (that can be hosted on different places, e.g. in a repository and as a blog post).
Only to clarify the state of accessibleAt: Do we want to allow more than one value or do we strictly limit it to a single value? If we limit it to a single value, should we introduce a new property that allows to point to other manifestations of an item?