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.
