Compute Eigenvalue of H2-Matrices by Slicing the Spectrum

[wawando]

Tasks of #77 Added Eigen_H2_Nlevel.cpp that computes some (or all) eigenvalue of a constant-rank H2-matrices in linear time (assuming constant spectrum length).

Additionals:

• Refactored commonly used classes in example files (functions, Particles, Box, and Domain) into examples/common directory
• Employ Ma's geometry partitioning based on SymmH2_ULV_SC22.cpp in examples/common/Domain.hpp
• Allow user to specify whether to use fixed-rank or fixed-accuracy in example files
• Fixed incorrect fill-in handling in existing H2-UMV and H2-LDL example files