Create output results directory if does not exists.
This fixes ROOT-5481
Apply fixes in copy ctor and assignment op. for Coverity
Add missing include file (unistd.h) defining ssize_t
Fix the PrintXML according to HistFactory.dtd  (InputFileLow/High -> HistoFileLow/High)
This fixes ROOT-5480
Fix stressHistfactory tests to build API model by passing correctly histoname and his to path.
In this way the model can be written correctly in an XML file.
Do come clean-up of code in PrintXML methods
Update documentation of Channel.h and Data.h update meaning of histoname and histopath