added ChecksumHelper

re #8924
re #8924 checkpointing work, but alg is functional
re #8924 first version complete with tests
re #8924 Addded rate limit handling
re #8924 improved error handling
re #8924  merge changes

Merge branch 'feature/8924_Intrument_file_repo' into develop

Conflicts:
	Code/Mantid/Framework/Kernel/inc/MantidKernel/ChecksumHelper.h
	Code/Mantid/Framework/Kernel/src/ChecksumHelper.cpp
	Code/Mantid/Framework/Kernel/test/ChecksumHelperTest.h