PMAC: Start up dls-pmac-control.py

Similarly to #9 , it would be nice if we could make our PMAC device look like a real PMAC to dls-pmac-control.py. As far as the PMAC is concerned, this should only require more dummy responses to certain commands used during startup and polling. However, since dls-pmac-control sends a PMAC commands as a single blob, e.g `#1PVF#2PVF` rather than `#1P V F #2 P V F`, a more complex tickit adapter will be required, https://github.com/dls-controls/tickit/pull/82 will no longer suffice. We will still want the device to work with EPICS, so commands must work with and without the whitespace present in EPICS messages.

In completing this, axis commands should be properly implemented, so solving this issue should also solve #10 .