Removing override, looks problematic with root

[juanangp]

After new changes in the code, it seems that root doesn't like override method, see below:

Fix this error by removing override from TRestVolumeHits

[nkx111]

You need to use ClassDefOverride instead ClassDef for cint, in case you have override keyword in the header.

This is not ROOT issue, this is a favor of gcc. Once you have one method marked with override, you need to mark override for all. On the other hand, ClassDef macro adds several methods implementing those from TObject, which in this case shall also be marked with override keyword. We can use ClassDefOverride macro to do this.