solver_callback
'''''''''''''''

In ``solver_callback``, it is shown how to write a solver callback function. It is done by writing a class that inherit
from ``InterceptSolverCallback``::

class SolverCallback(aka.InterceptSolverCallback):
[.....]

solver_callback = SolverCallback(model)

model.solveStep(solver_callback)

In that case, the solver callback is used to modify the stiffness matrix.