Implemented yz-implicit diffusion. #18

Job	Run time
NVIDIA / build-opt	2m 53s
INTEL / build-opt	1m 0s
INTEL / build-dbg	48s
NVIDIA / build-dbg	2m 43s
INTEL / build-opt-omp	1m 0s
NVIDIA / build-opt-omp	2m 39s
GNU / build-opt-omp	42s
GNU / build-dbg	33s
GNU / build-opt	48s
GNU / test-opt-differentially_heated_cavity	1m 36s
GNU / test-opt-lid_driven_cavity	25s
GNU / test-dbg-lid_driven_cavity	51s
GNU / test-dbg-differentially_heated_cavity	9m 46s
INTEL / test-dbg-lid_driven_cavity	1m 6s
INTEL / test-opt-differentially_heated_cavity	4s
INTEL / test-opt-lid_driven_cavity	3s
INTEL / test-dbg-differentially_heated_cavity	5m 8s
NVIDIA / test-opt-differentially_heated_cavity	2m 10s
NVIDIA / test-opt-lid_driven_cavity	2m 16s
NVIDIA / test-dbg-lid_driven_cavity	3s
NVIDIA / test-dbg-differentially_heated_cavity	3s
	36m 37s

Provide feedback