Improve the solver by following

  1. Previous version won't work when the bank voltage is over 10V
     , which is the upper bound I set for the solver.
     So the solver will never find a solution when bank voltage is
     over 10V.
     There should be no reason for this restriction, so I release
     the upper bound to a 1000V.

     This make the secondary initial guess starts from 1000V. So I
     use the number min(ub, 50). When DC given input, this number is
     50. When DC given output, this number is the bank voltage.

   2. I also relaxed the delta to 2e-3, when sundials solver returns
      in case max iteration arrived (2). Otherwise, when load power
      is 0.8, it just stucks some where.