From Gerri:
Add the following 'merge' modes:

    'M'        final value will be the maximum
    'm'        final value will be the minimum
    'f'        final value will be the first encountered
    'l'        final value will be the last encountered

Defined also a new bit 'kIsConst' which is set if all the 'merged'
values are found to be the same; a new method 'Bool_t IsConst() const'
is provided to quickly test constancy.

Should provide a solution for the requirements raised in Savannah bug
#95957 and Savannah patch #5550.


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@49118 27541ba8-7e3a-0410-8455-c3a389f83636