some change on the variable name and fix a tiny bug

- CHG : change the name of some class members in tunnel-related solver
- FIX : fig a bug in the formula to calculate tunneling current density
- ENH : change the method to fill the trap vertex in the tunnel solver
add band-to-trap tunneling mechanism, but does not work correctly

- NEW : add the method for band-to-trap tunneling in the trap solver and tunnel solver
- ENH : enhance the method for calculating the transmission coefficient
- CHG : change the method for considering the trapping vertex in the tunneling solver
debug band-to-trap tunneling

- FIX : fix a bug in calculating the band-to-trap tunneling coefficient
- ENH : separate MFN and B2T tunneling
add the switch on MFN tunneling