More work on killing jobs

Answers.py:
-documentation changes
-introduced an answer for KillCurrentJobQuestion
Questions.py:
-fixed an error in CMDQuestion where the import was missing for
Hydra_rendertask
-documentation changes
-added exception for subclassing Question without implementing
computeAnswer
-added safeguarding to KillCurrentJobQuestion to prevent it being run on
non-TCPServers
-KillCurrentJobQuestion now returns KillCurrentJobAnswer
RenderNodeMain.py:
-changed killCurrentJob method to check if the subprocess was actually
terminated before setting childKilled to True.
-moved line "self.childProcess = None" so that it executes as late as
possible because I'm paranoid about a race condition in which
childProcess gets set to None before the killCurrentJob method is able
to ask about its returncode but too lazy to set up a mutex to completely
prevent it if that's even necessary.
Clients.py:
-No changes
JobKill.py:
-implemented sendKillQuestion
getOff.py:
-introduced "killed" to accept the answer from the RenderNode, with a
debug line if the job wasn't killed for some reason