Add CellIFT mod #14

Job	Run time
test-linux (ubuntu-20.04, focal, clang-12, c++11)	11m 38s
test-linux (ubuntu-20.04, focal, clang-12, c++14)	12m 44s
test-linux (ubuntu-20.04, focal, clang-12, c++17)	12m 25s
test-linux (ubuntu-20.04, focal, clang-12, c++20)	12m 7s
test-linux (ubuntu-20.04, focal, gcc-11, c++11)	13m 9s
test-linux (ubuntu-20.04, focal, gcc-11, c++14)	11m 11s
test-linux (ubuntu-20.04, focal, gcc-11, c++17)	11m 4s
test-linux (ubuntu-20.04, focal, gcc-11, c++20)	11m 7s
test-linux (ubuntu-20.04, focal, clang-11, c++11)	12m 34s
test-linux (ubuntu-20.04, focal, gcc-10, c++11)	10m 25s
test-linux (ubuntu-18.04, bionic, clang-3.9, c++11)	1s
test-linux (ubuntu-18.04, bionic, gcc-4.8, c++11)	1s
	1h 58m 26s

Provide feedback