Update Dmeson single particle gun pt-eta distribution
This updates the pt-eta distribution for the D meson single particle gun samples (902066, 902067, 902068) added by !3163 (merged).
These samples have not been produced yet so this update doesn't affect any datasets.
The eta cut is changed from |eta|<2.2 to |eta|<2.5.
pt between 5 and 8 GeV is added, using a flat shape (~10% of total events) to avoid having too many events at low pt.
