Updated the help files to describe the new preferences panel
Added some error handling to the CombineFiles? panel.
-This line, and those below, will be ignored--
M trunk/NCNR_User_Procedures/Reduction/SANS/NSORT.ipf
M trunk/NCNR_Help_Files/SANS_Reduction_Help_v5.20.ihf
M trunk/NCNR_Help_Files/SANS_Analysis_Docs_v4.10.ihf
M trunk/NCNR_Help_Files/USANSReductionHelp_v2.3.ihf