[RooFit] Add missing 'add' functions to RooArgSet.

[riga]

This PR adds 'add' functions to RooArgSet again, that were removed in 6.06/00.

Problem

This pyROOT code

import ROOT
set1 = ROOT.RooArgSet()
set2 = ROOT.RooArgSet()
set1.add(set2)

worked until 6.05/02 (included) and raises an exception since 6.06/00:

TypeError: bool RooArgSet::add(const RooAbsArg& var, bool silent = kFALSE) =>
    could not convert argument 1

Fix

RooArgSet inherits from RooAbsCollection, so it should be possible to add a RootArgSet instance to an other RootArgSet instance via add(). This used to work in versions prior to 6.06/00. From RooArgSet.h in 6.05/02:

class RooArgSet : public RooAbsCollection {
public:
    ...
    virtual Bool_t add(const RooAbsCollection& list, Bool_t silent=kFALSE) {...}
};

This was changed in commit 283f080, where e.g. the above add function was replaced by

using RooAbsCollection::add;

However, this only seems to work when the used function is implemented in the base class' header or re-implemented by the inheriting class' which both is not the case. Therefore, I added the functions again to the RooArgSet header.

[peremato]

Thanks for the patch. This fixes issue https://sft.its.cern.ch/jira/browse/ROOT-8230
Merged to master and 6.08 branches.