[hist] Fix I/O of TFormula nd TFitResultPtr

[lmoneta]

Fix the I/O of predefined functions (gaus, expo, etc..) where the data member fNumber was not saved in the file and could not be recomputed afterwards.

Fix also the I/O of the TFitResultPtr class

This PR fixes #16184

[github-actions[bot]]

Test Results

    13 files      13 suites   2d 19h 19m 8s :stopwatch:  3 025 tests  2 999 :white_check_mark: 0 :zzz: 26 :x: 33 786 runs  33 701 :white_check_mark: 0 :zzz: 85 :x:

For more details on these failures, see this check.

Results for commit cabac170.

[dpiparo]

On the forum, another problem was identified, which was described in the original ticket.