Draft: Saddle point solver

Solves the following issue(s):

Closes #...

Core changes:

In mass.py, M2B was renamed to M2b_div0, and new M2B uses weights directly.

Model-specific changes:

Add model Stokeslike that follows paper "Finite element discretization of a Stokes-like model arising in plasma physics". The governing equations are given by: \begin{gather} \partial_t u = - \nabla \phi + u \times B + \nu \Delta u + f \,,\\ 0 = \nabla \phi- u_e \times B + \nu_e \Delta u_e + f_e \,, \\ \nabla \cdot (u-u_e) = 0 \,. \end{gather} for u \in H(div), u_e \in H(div), \phi \in L^2

Uses new propagator "Stokes" and axisymmetric MHD equilibrium "FluxAlignedTokamak" is implemented according to the reference paper.

Documentation changes:

None