add option to solve discrete rather than exact laplacian #559

Job	Run time
GNU@7.5 C++14 [tutorials]	10m 40s
Clang@6.0 C++14 SP Particles DP Mesh Debug [tutorials]	7m 21s
GNU@10.3 C++20 OMP [tutorials]	11m 2s
CUDA@11.2 GNU@9.3.0 C++17 Release [tutorials]	30m 12s
DPCPP GFortran@7.5 C++17 [tutorials]	29m 3s
HIP ROCm GFortran@9.3 C++17 [tutorials]	1h 4m 43s
GNU@7.5 C++14 w/o Fortran [tutorials]	8m 38s
GNU@7.5 C++14 NOMPI [tutorials]	10m 27s
	2h 52m 6s

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