some bug fixed and inspect the DD matrix and rhs

- FIX : fix the coefficient filling problem at boundary of trapping layer and oxide layer
- FIX : use normalized value when initializing the material properties.
- FIX : when setting the length of the structure, excess nm_in_cm factor is used. fix this bug.
- NEW : add the preparation and initialization methods in DD test
- CHG : change the method for initializing the normalization factors
- ENH : add some comments in SctmUtils
- ENH : enhance the time step generator