Start sequencer

Add a python program for starting the various services needed on the front end: models, dolphin drivers, daq streamers. The program also handles shut down.

For the sites, this an improvement over the complicated and large mess of system.d units and overrides that currently control the startup. The sequence for both startup and shutdown is defined in FrontEndStateMachine.py, and the interface with dbus, a communication bus with systemd, is in FrontEndDriver.py. Both are relatively small straightforward Python. Also CDSRFMStateMachine.py give the sequence for CDSRFM servers.

Some draw backs: the sites will lose rts-models, rts-user-models, and rts-transport targets. These were convenient shortcuts. The sequencer can be easily improved to provide the same sort of shortcuts.

CyMACs get a working startup system for models right out of the package. Depending on the configuration there will be some environment variables that must be defined.

There is an associated packaging MR, number to be given in comments, targeting the debian/buster branch. Packaging will be broken if one MR is accepted but not the other.