[RF] Don't use `RooArgSet::operator new` in derived classes
[RF] Don't use `RooArgSet::operator new` in derived classes

Before this PR, any class derived from RooArgSet that was not purely an
interface class had to reimplement `operator new`.

By checking for the requested memory size, one can make sure directly in
`RooArgSet::operator new` that the derived classes don't use the custom
implementation.

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