Unify the H2-matrix data structure throughout Hatrix.

[v0dro]

There are various definitions of H2-matrix in the examples. We first unify them across example files and then move to https://github.com/rioyokotalab/Hatrix/issues/112.

Current various (bifurcated) implementations of H2-matrices:

From @v0dro :

SymmetricSharedBasisMatrix:

• [x] https://github.com/rioyokotalab/Hatrix/blob/dev/examples/distributed/SymmetricSharedBasisMatrix.cpp
• [x] https://github.com/rioyokotalab/Hatrix/blob/dev/examples/distributed/include/distributed/SymmetricSharedBasisMatrix.hpp
• [ ] https://github.com/rioyokotalab/Hatrix/blob/dev/examples/H2_strong_CON.cpp

From @wawando :

SymmetricH2:

• [x] https://github.com/rioyokotalab/Hatrix/blob/dev/examples/H2_strong_CON_sym_ID.cpp
• [ ] https://github.com/rioyokotalab/Hatrix/blob/dev/examples/H2_strong_EV_sym.cpp
• [ ] https://github.com/rioyokotalab/Hatrix/blob/dev/examples/H2_strong_EV_sym_mpi.cpp
• [ ] https://github.com/rioyokotalab/Hatrix/blob/dev/examples/H2_strong_EV_sym_mpi_ref.cpp
• [ ] https://github.com/rioyokotalab/Hatrix/blob/dev/examples/H2_strong_EV_sym_nodep.cpp
• [x] https://github.com/rioyokotalab/Hatrix/blob/dev/examples/H2_strong_LDL.cpp
• [x] https://github.com/rioyokotalab/Hatrix/blob/dev/examples/H2_strong_LDL_nodep.cpp
• [x] https://github.com/rioyokotalab/Hatrix/blob/dev/examples/H2_strong_PO.cpp
• [x] https://github.com/rioyokotalab/Hatrix/blob/dev/examples/H2_strong_PO_nodep.cpp

[v0dro]

Order of execution:

1. https://github.com/rioyokotalab/Hatrix/issues/114
2. https://github.com/rioyokotalab/Hatrix/issues/115