add ec:replaces for Resource (just like for Asset)

In the schema an Asset can be replaced by another.
It would make sense if a Resource can also be replaced by another.

```
ec:Resource
...
[
    a owl:Restriction ;
    owl:onProperty ec:isReplacedBy ;
    owl:allValuesFrom ec:Resource
  ], [
    a owl:Restriction ;
    owl:onProperty ec:replaces ;
    owl:allValuesFrom ec:Resource
  ],```
