When invoking SiteCatalog.write(), TransformationCatalog.write(), and ReplicaCatalog.write(), the write location should be stored as a member variable in those instances as this location can be later used, for example, when specifying pegasus.catalog.replica.file.

This is also consistent with the Workflow implementation, as we store its path upon a call to wf.write().

Example.
sc = SiteCatalog()
sc.write("/here/is/sites.yml")
p = sc.location() # returns /here/is/sites.yml