Diffusion: tests, fixed coefficients in the solver, & tutorial #859

Job	Run time
CMake Matrix Test. (Linux Min GCC, ubuntu-22.04, gcc-12, g++-12, 3.9, 3.19.x, ON, OFF, static)	14m 58s
CMake Matrix Test. (Linux Min GCC, ubuntu-22.04, gcc-12, g++-12, 3.9, 3.19.x, ON, OFF, shared)	11m 3s
CMake Matrix Test. (Linux Min Clang, ubuntu-22.04, clang-13, clang++-13, 3.9, 3.19.x, ON, OFF, st...	8m 7s
CMake Matrix Test. (Linux Min Clang, ubuntu-22.04, clang-13, clang++-13, 3.9, 3.19.x, ON, OFF, sh...	8m 11s
CMake Matrix Test. (MacOS Min, macos-13, clang, clang++, 3.9, 3.19.x, ON, OFF, static)	15m 6s
CMake Matrix Test. (MacOS Min, macos-13, clang, clang++, 3.9, 3.19.x, ON, OFF, shared)	11m 50s
CMake Matrix Test. (Linux Max GCC, ubuntu-24.04, gcc-14, g++-14, 3.12, 3.22.x, ON, OFF, static)	16m 13s
CMake Matrix Test. (Linux Max GCC, ubuntu-24.04, gcc-14, g++-14, 3.12, 3.22.x, ON, OFF, shared)	13m 14s
CMake Matrix Test. (Linux SIMD, ubuntu-24.04, gcc-14, g++-14, 3.12, 3.22.x, OFF, ON, static)	14m 44s
CMake Matrix Test. (Linux SIMD, ubuntu-24.04, gcc-14, g++-14, 3.12, 3.22.x, OFF, ON, shared)	12m 33s
CMake Matrix Test. (Linux Max Clang, ubuntu-24.04, clang-18, clang++-18, 3.12, 3.22.x, ON, OFF, s...	13m 56s
CMake Matrix Test. (Linux Max Clang, ubuntu-24.04, clang-18, clang++-18, 3.12, 3.22.x, ON, OFF, s...	10m 32s
CMake Matrix Test. (MacOS Max, macos-14, clang, clang++, 3.12, 3.22.x, ON, OFF, static)	15m 3s
CMake Matrix Test. (MacOS Max, macos-14, clang, clang++, 3.12, 3.22.x, ON, OFF, shared)	10m 16s
	2h 55m 46s

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