PT-Isochrones with different access-egress modes.

[idegeus]

Describe the bug
I'm working to get a PT-isochrone on /isochrone-pt/ with different access and egress modes, like implemented in the normal /route endpoint. Currently, adding anything to modify access/egress but pt.earliest_departure_time to /isochrone-pt/ is not supported.

To Reproduce
I am using the latest build of Graphhopper, and I made some modifications to PtRouteResource.java, inspired by PtRouterImpl.java

Expected behavior
It seems like the current implementation also does not support different access/egress modes. That would be good.

Steps taken
I have taken some first shot at this in #2855, but Java is not my native language, so I imagine there are inherently some errors. Would be great if someone took a look!