add criterion for checking steady state and accordingly fix the voltage sweep approach

[prkkumar]

This PR implements a criterion for checking if the steady-state has reached:

(Phi_{n} - Phi_{n-1}).norm0() < phi_tolerance

Default value of phi_tolerance = 1.e-7.

It can be overwritten by the user in the input script.

Test input script: inputs_mfim_2d.txt

Screenshot of stdout at the end:

Simulation reaches steady state at step = 1067 when error = 9.971617631e-08

Final plotfile written is plt00001067.