Draft: Allow the Roi default zed width to be modified

We don't want this to be changed easily all the time - this would be quite fragile so we allow it to be set once and once only.

In principle we would want it to only be setable before any RoiDescriptors are created, which could be done by incrementin s_icount also in the RoiDescriptor::construct() method, but this would require a mutex call to increment the counter for the construction of any RoiDescriptor which would come with a performance penalty so has been missed out. I have a mechanism that could be used to achioeve this with no penalty, but that is for a new request.