fixed #197

Instead of checking each RetrievalServiceLayer if it has a PNode-Method
the HeadlessMapProvider checks now for the instances of PNodeProvider,
which is exactly what it is made for.
Merge branch 'bug/197-headlessmapprovider-checkforpnodeprovider' into dev
Merge branch 'dev' of github.com:cismet/cismap-commons into dev