add option to solve discrete rather than exact laplacian #555

Job	Run time
[email protected] C++14 [tutorials]	11m 4s
[email protected] C++14 SP Particles DP Mesh Debug [tutorials]	7m 6s
[email protected] C++20 OMP [tutorials]	11m 11s
[email protected] [email protected] C++17 Release [tutorials]	33m 21s
DPCPP [email protected] C++17 [tutorials]	29m 17s
[email protected] C++14 w/o Fortran [tutorials]	9m 53s
[email protected] C++14 NOMPI [tutorials]	9m 18s
HIP ROCm [email protected] C++17 [tutorials]	1h 4m 36s
	2h 55m 46s

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