separate the method for calculating slice flatband voltage and domain flatband voltage

- NEW : the domain flatband voltage is now computed using the weighted-average value of slice flatband voltage at each channel vertex with respect to its control length
- BUG FIND : find a bug in calculating the substrate potential and fermi level above Ec. In current method, all the values of channel vertex are same, but should be different in 2D case.
enhance the substrate solver

- ENH : the substrate solver now calculate the channel potential and fermi energy above conduction band for each channel vertex.
- CHG : change the method for fetching result of substrate solver in solver pack
- ENH : enhance the method to write substrate result in SctmData
Merge branch 'reviseSubsSolver'