Diffusion: tests, fixed coefficients in the solver, & tutorial #860

Job	Run time
CMake Matrix Test. (Linux Min GCC, ubuntu-22.04, gcc-12, g++-12, 3.9, 3.19.x, ON, OFF, static)	6h 0m 22s
CMake Matrix Test. (Linux Min GCC, ubuntu-22.04, gcc-12, g++-12, 3.9, 3.19.x, ON, OFF, shared)	6h 0m 24s
CMake Matrix Test. (Linux Min Clang, ubuntu-22.04, clang-13, clang++-13, 3.9, 3.19.x, ON, OFF, st...	6h 0m 16s
CMake Matrix Test. (Linux Min Clang, ubuntu-22.04, clang-13, clang++-13, 3.9, 3.19.x, ON, OFF, sh...	6h 0m 15s
CMake Matrix Test. (MacOS Min, macos-13, clang, clang++, 3.9, 3.19.x, ON, OFF, static)	24m 12s
CMake Matrix Test. (MacOS Min, macos-13, clang, clang++, 3.9, 3.19.x, ON, OFF, shared)	11m 7s
CMake Matrix Test. (Linux Max GCC, ubuntu-24.04, gcc-14, g++-14, 3.12, 3.22.x, ON, OFF, static)	6h 0m 24s
CMake Matrix Test. (Linux Max GCC, ubuntu-24.04, gcc-14, g++-14, 3.12, 3.22.x, ON, OFF, shared)	6h 0m 26s
CMake Matrix Test. (Linux SIMD, ubuntu-24.04, gcc-14, g++-14, 3.12, 3.22.x, OFF, ON, static)	15m 3s
CMake Matrix Test. (Linux SIMD, ubuntu-24.04, gcc-14, g++-14, 3.12, 3.22.x, OFF, ON, shared)	15m 12s
CMake Matrix Test. (Linux Max Clang, ubuntu-24.04, clang-18, clang++-18, 3.12, 3.22.x, ON, OFF, s...	6h 0m 19s
CMake Matrix Test. (Linux Max Clang, ubuntu-24.04, clang-18, clang++-18, 3.12, 3.22.x, ON, OFF, s...	6h 0m 17s
CMake Matrix Test. (MacOS Max, macos-14, clang, clang++, 3.12, 3.22.x, ON, OFF, static)	15m 12s
CMake Matrix Test. (MacOS Max, macos-14, clang, clang++, 3.12, 3.22.x, ON, OFF, shared)	8m 55s
	2d 1h 32m 24s

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