HVCAEN: make number of summary channels configuarable

[FreddieAkeroyd]

The PR ISISComputingGroup/EPICS-HVCAENx527#16 introduced a NCHANSUMM macro which is currently defaulted at 200. This should be made available via the IOC as a configuration macro. There is a second derived macro NCHANSUMMCHAR that is needed to set the size of a waveform char buffer, this should be calculated in the ioc st.cmd to be 40 * $(NCHANSUMM) and then passed to the Db as NCHANSUMMCHAR -(NCHANSUMMCHAR should not be a configuration user settable macro)