[RF] added new printout and new constructors to RooCmdArg

[cburgard]

This Pull request:

• Adds new Constructor interfaces to RooCmdArg
• Adds new printouts to RooCmdArg

Changes or fixes:

RooCmdArg is a bit of an old-style piece of code that doesn't really work well with python. Also, when talking among statistics code developers, it's commonplace to have to "exchange" fit arguments between codes ("what arguments to you pass to make it converge?"). For this purpose, it's very convenient to:

• be able to print the command arguments in a human-readable format, and
• directly use these printouts to copy&paste them into some other piece of code to make comparison studies

The changes in this PR make this possible with little effort.

Checklist:

• [x] tested changes locally
• [ ] updated the docs (if necessary)

This PR fixes #

[dpiparo]

Thanks @cburgard . I started the tests and leave the review in the hands of @guitargeek

[github-actions[bot]]

Test Results

    11 files      11 suites   2d 15h 53m 4s :stopwatch:  2 615 tests  2 613 :white_check_mark: 0 :zzz: 2 :x: 28 499 runs  28 497 :white_check_mark: 0 :zzz: 2 :x:

For more details on these failures, see this check.

Results for commit f5bae329.

:recycle: This comment has been updated with latest results.