Fix the use of string in TTree's alias and TTreeFormula by

fixing the detection that a TTreeFormula is a string or not.

Move to using kIsCharacter only as an accelerator to TTreeFormula::IsString
and always use this function to detect whether the formula is a string or not.
Fix (in TTreeFormula::Init) several cases of string detection and remove from
TFormula any settings (they were wrong anyway, they should have been ResetBit
rather than SetBit) of kIsCharacter.


git-svn-id: http://root.cern.ch/svn/root/trunk@43508 27541ba8-7e3a-0410-8455-c3a389f83636