NIRB implementation with Feelpp

[alielarif]

Make the implementation of NIRB methods by using Feelpp environment

[alielarif]

We use feelpp to compute initial Snapshots and to generate interpolation operator. However, to generate reduced basis, we used numpy linear algebra to solve eigen value problem, so that the reduced basis matrix is in numpy format. To perform the computation, I would like to :

• find a way to solve eigen value problem with feelpp solver (to be able to use feelpp environment for all computation)
• how to save petsc matrix on a disk and load them in the same format (I need to save mass matrix and reduced basis matrix from Offline part in Ptsc format and load them in Online part)

[prudhomm]

@alielarif we do have eigenvalue solvers for dense and sparse problems. Slepc is the library (using petsc) to do this for eigen solves. We have a function called eigs() that will do the job. See eg https://github.com/feelpp/feelpp/blob/develop/testsuite/feelalg/test_eigenmode.cpp