NormaliseToMonitor needs to support events

[martyngigg]

Original Owner: @jzikovsky

Hi Nick,

I am having issues with NormalizeToMonitor from an eventmode data.
I am reading event workspace (that creates 2 wk: data and monitors), rebinning both, then extracting a single spectrum (still have to do that as far as I can see, I reported that ages ago) and normalizing the data. This used used to work before but now I get this :
Error in execution of algorithm NormaliseToMonitor:
EventWorkspace::dataY cannot return a non-const array: you can't modify the histogrammed data in an EventWorkspace!

[martyngigg]

This issue was originally trac ticket 4720