[RF] Handle variables having numeric value as name in RooFormula

[Ban42]

This Pull request:

As requested in #11992 changes the way RooFormulaVar performs variable names substitutions.

Changes or fixes:

Before performing a substitution in the formula based on variable name, the code now checks if the variable to be substituted has a number as name.
Variables where the name is a number are now only be matched by index. If there is a variable with a name that is a number that is not a RooConstVar where the value is equal to the name, an exception will be thrown.

Checklist:

• tested changes locally: Yes, 227 - gtest-roofit-roofitcore-testRooFormula fails, throwing one of the two exceptions I have created. I either have to update it, or double-check for mistakes on my side.
• updated the docs : Let me know if I should.
• Add tests: I probably need to add a test for this

This PR fixes #11992

[Ban42]

Hi @guitargeek ,
This is the solution I have found which should match the specification you described in #11992

The test "227 - gtest-roofit-roofitcore-testRooFormula" fails, which means I have made a mistake or I have to update it. Before doing so, I wanted you to have a quick look and confirm this approach is the desired one.

Thank you :)

[dpiparo]

let's start with the CI, to see where we stand.