Forbid changing the job.server.run_mode after job.run

[pmrv]

Over in #1155 we discussed that it's quite weird that we allow to change the run mode of a job after it has or while it is running. Refreshed and full of energy I went this morning to fix this---only to discover why we don't disallow this yet. Server doesn't have a back reference to the job and can therefor not check its status in the run_mode setter. But actually you don't really want to change anything about the server object after the job is run, so alternatively we could add a freeze method to the server, that the job calls on run, after which no attribute modifications are allowed any more. Special considerations would have to be taken for the future object of #1155, because that one we do want to change after the fact, but that's ok imo. Thoughts?

[jan-janssen]

I like the idea, in principle every object attached to the job should have this option, as well as an unfreeze option when somebody copies only the input of a given job to run it again.

[pmrv]

I've started working on this in #1162.