Fix Issue #16
Increase PoissonSolver tolerance in inducedfield_* and fdtd/* tests and fix PoissonSolver restart issue in QSFDTD.
With increased tolerance the tests should pass with tol = 1e-6. (Currently tol = 0.001 in inducedfield_td.py is really large.)
I haven't tested if this fixes the issue on Mac, so please test it.