[RF] Fix ambiguity when calling RooDataHist constructor in PyROOT

The RooDataHist has two similar constructors:

```C++
RooDataHist(RooStringView name, RooStringView title, const RooArgSet& vars, const char* binningName);
RooDataHist(RooStringView name, RooStringView title, const RooArgList& vars, const RooCmdArg& arg1);
```

The problem: a `RooCmdArg` can be implicitly constructed from a `const
char*`. Therefore, if one doesn't explicitly choose one of the two
overloads by passing a RooArgSet or RooArgList as the third argument
explicitly, both overloads are succeeding in PyROOT. Unlike in C++,
the overload that doesn't require implicit conversion is not
prioritized.

Here is an example of where this goes wrong:

```Python
ROOT.RooDataHist(name, title, variables, "my_binning")
```

...causing...

```
[#0] ERROR:InputArguments -- RooDataHist::ctor() ERROR: unrecognized command: my_binning
```

...because the constructor with the RooCmdArg is used.

It was sheer luck that this problem didn't appear so far. It only
appeared after the the constructors were changed to use `RooStringView`,
probably because the order is which the overloads are tried is not well
defined.

The solution: make the implicit construction of `RooCmdArg` from just a
string impossible. This is done by requiring at least one non-default
payload parameter. This is not breaking any code, because constructing a
RooCmdArg without any payload doesn't make sense anyway.

Author: guitargeek

roofit/roofitcore/inc/RooCmdArg.h

@@ -27,8 +27,14 @@ class RooCmdArg final : public TNamed {
 public:
 
   RooCmdArg();
+  /// Constructor from payload parameters. Note that the first payload
+  /// parameter has no default value, because otherwise the implicit creation
+  /// of a RooCmdArg from `const char*` would be possible. This would cause
+  /// ambiguity problems in RooFit code. It is not a problem that the first
+  /// parameter always has to be given, because creating a RooCmdArg with only
+  /// a name and no payload doesn't make sense anyway.
   RooCmdArg(const char* name, 
-	    Int_t i1=0, Int_t i2=0, 
+	    Int_t i1, Int_t i2=0,
 	    Double_t d1=0, Double_t d2=0, 
 	    const char* s1=0, const char* s2=0, 
 	    const TObject* o1=0, const TObject* o2=0, const RooCmdArg* ca=0, const char* s3=0,