Diffusion: tests, fixed coefficients in the solver, & tutorial #847

Job	Run time
CMake Matrix Test. (Linux Min GCC, ubuntu-22.04, gcc-12, g++-12, 3.9, 3.19.x, ON, OFF, static)	15m 14s
CMake Matrix Test. (Linux Min GCC, ubuntu-22.04, gcc-12, g++-12, 3.9, 3.19.x, ON, OFF, shared)	11m 49s
CMake Matrix Test. (Linux Min Clang, ubuntu-22.04, clang-13, clang++-13, 3.9, 3.19.x, ON, OFF, st...	8m 34s
CMake Matrix Test. (Linux Min Clang, ubuntu-22.04, clang-13, clang++-13, 3.9, 3.19.x, ON, OFF, sh...	8m 38s
CMake Matrix Test. (MacOS Min, macos-13, clang, clang++, 3.9, 3.19.x, ON, OFF, static)	17m 2s
CMake Matrix Test. (MacOS Min, macos-13, clang, clang++, 3.9, 3.19.x, ON, OFF, shared)	23m 28s
CMake Matrix Test. (Linux Max GCC, ubuntu-24.04, gcc-14, g++-14, 3.12, 3.22.x, ON, OFF, static)	18m 31s
CMake Matrix Test. (Linux Max GCC, ubuntu-24.04, gcc-14, g++-14, 3.12, 3.22.x, ON, OFF, shared)	14m 2s
CMake Matrix Test. (Linux SIMD, ubuntu-24.04, gcc-14, g++-14, 3.12, 3.22.x, OFF, ON, static)	15m 16s
CMake Matrix Test. (Linux SIMD, ubuntu-24.04, gcc-14, g++-14, 3.12, 3.22.x, OFF, ON, shared)	11m 38s
CMake Matrix Test. (Linux Max Clang, ubuntu-24.04, clang-18, clang++-18, 3.12, 3.22.x, ON, OFF, s...	14m 24s
CMake Matrix Test. (Linux Max Clang, ubuntu-24.04, clang-18, clang++-18, 3.12, 3.22.x, ON, OFF, s...	11m 9s
CMake Matrix Test. (MacOS Max, macos-14, clang, clang++, 3.12, 3.22.x, ON, OFF, static)	17m 3s
CMake Matrix Test. (MacOS Max, macos-14, clang, clang++, 3.12, 3.22.x, ON, OFF, shared)	9m 27s
	3h 16m 15s

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