[RF] Allow to set nbins for RooPlot (or disallow)

[ynikitenko]

Is your feature request related to a problem? Please describe.

I want to create a frame for a RooFormulaVar. Unfortunately, it doesn't have a method frame(), so I wanted to use a RooPlot constructor to create the frame, and later use the existing plotOn() method of RooFormulaVar.

However, it seems that RooPlot allows setting the number of bins only for RooAbsRealLValue, which is inconsistent. No method to set the number of bins is available. The underlying access to nbins is private.

Describe the solution you'd like

Allow to set the number of bins in all constructors.

Maybe there should be a separate method for that, but it may be dangerous if someone tries to change the number of bins in the middle of processing.

Describe alternatives you've considered

I already decided to use a new RooRealVar instead of RooFormulaVar, but I think that a plot should allow to set the number of bins.

UPD: no, there are no arguments to change the number of bins in plotOn. It seems it can be set only for the Plot.
It looks like I can set the number of bins during plotOn of RooFormulaVar. This is great. However, I think that in this case the number of bins in the constructor should be removed, to reduce code duplication. At least it should be made consistent in all constructors (see no reason for the otherwise, but I don't understand RooFit well yet).

P.S. There is also a typo in the RooPlot class description, "settting".

[ynikitenko]

It seems that to plot RooFormulaVar is not very straightforward (https://root-forum.cern.ch/t/how-to-plot-a-rooformulavar/26480/2), and in that way it is possible to set the number of bins.
However, I'm not sure how this is related to this feature request (maybe to set the number of bins might be useful for other cases?)