add TAT mechanism for electrons

- NEW : add the class and method for TAT mechanism
select the TAT model

- NEW : tried several methods to consider TAT...
get ready to demo the TAT effect

- NEW : add the method to write time constant for TAT
consider tunneling direction in TAT tunneling solver

- NEW : consider tunneling direction when calculating the TAT current
- FIX : fix a bug in calculation of the time constant
TAT can work in retention

- FIX : fix a bug in calculating the time constant when electrons tunnel from trap layer to subs (energy difference)
- FIX : fix a bug in calculating time constant when electrons tunnel from subs to trap layer (normalize the energy)
Trap-assisted trap-to-band tunneling can work now

- NEW : add new methods for trap-assisted trap-to-band tunneling mechanism
- CHG : change the input parameters in the TAT solver
tiny revision for testing