Updates README
Updates DCSolver.*pp

  Change the rule when deciding if a solution from sundials solver
  is correct or not.
  Previously, it is valid only when sundials returns KIN_SUCCESS.
  Now I relax to when the amount
  |power_dcdc + power_vout - power_vin| < 1e-3.
  This fixed the bug, when I use 1.6 load power to charge, it charge
  even less than use 2.0 load power.

  You can use last commit to try, give the task length like 2000.