Making ex18 much much faster (Experimental, Do-Not-Merge)

[sshiraiwa]

This is an experimental branch to expand Numba compilation in PyMFEM. It uses numba-swig-bridge (NSB) so that MFEM object can be used inside Numba's @njit section. NBS-bridged director also allows for using Numba-compiled method to override a method. Consequently, DGHyperbolicConservationLaws::MULT in ex18 can run much much faster, even if it is written in Python. Benchmark indicates the wall clock time can be close to the C++ example. (The pure Python code is ~x20 slower than this.)

ToDo:

• ex18 : acceleration by Numba-compiled director callback
• ex4 : MSB version of Coefficeint
• ex38
• ex9
• ex19
• parallel versions (ex4p, ex9p, ex18p, ex19p + nsb_artifacts)