Navier miniapp

[andersonw1]

This PR compiles PyMFEM with the navier miniapp by including the interface file navier_solver.i. The input file navier_mms.cpp is also converted to python as an example.

[chldkdtn]

@sshiraiwa if you can review and merge this to master branch, that would be wonderful.

[sshiraiwa]

@chldkdtn , @andersonw1, Thank you for contribution. The CI error doesn't seems caused by this PR itself. I am looking into it.

[sshiraiwa]

navier_solver.hpp contains the following methods, which takes C function pointer.
'''
void AddVelDirichletBC(VecFuncT *f, Array &attr);
void AddPresDirichletBC(ScalarFuncT *f, Array &attr);
void AddAccelTerm(VecFuncT *f, Array &attr);
'''
For Python, these methods do not make much sense, and in fact, causes build error.
I proposed that we ignore these methods using %ignore, since Python user can use Numba compiled coefficient to call these methods, as you are doing in navier_mms.py

[sshiraiwa]

Please let us know if the suggested change is okay for you. If it is okay, we will merge it. Thank you for fantastic contribution!