Extend Pmac Trajectory to work for compound motors

[tomtrafford]

          > I believe this would only work for compound motors. 

Correct

An Axis with straight through mapping, (p45 Sample X for example) does not have a compound motor and its link is '@ASYN(PORT, num)' where PORT is the brick and num refers to its axis number on the brick rather than a CS assignment.

I forgot P45 didn't have all compound axes... I guess we'll have to solve this now.

However from the brick and axis number I can then use '{PMAC_PREFIX}:M{num}:CsPort_RBV' and '{PMAC_PREFIX}:M{num}:CsAxis_RBV' to get the port and axis. But this feels quite long-winded...

Yes, that seems right. However I think I did the wrong thing in Malcolm attaching them to the motor. We should attach them to the generic PMAC object instead, in the same way as the EDM screens have an "Axes" table with deferred move, CS port and CS Axis.

I think we need to discuss this with the DeviceVector part below, but for now could you attach these somewhere to the PMAC object so you can use them in the CS code above?

Originally posted by @coretl in #440 (comment)