Expose commands within the frameProcessor

[GDYendell]

To support e.g. start writing, stop writing in the hdf plugin. We would add a new IpcMsgVal for commands (MsgValCmdCommand), add command and request_commands methods to FrameProcessorController / FrameProcessorPlugin and then it should just be sent to the correct plugin based on the first part of the path (e.g. "hdf") and then only that plugin should have to handle the command subsystem.

request_commands should return a dictionary of command name to what parameters it takes, or None / empty dict if it does not take any parameters.
command should take a command name and a dict of parameters (again empty / not given if it does not take parameters)

[ajgdls]

See PR

#359