Allow dipole correction in work function calculation

[ahxbcn]

Dipole correction is allowed in the tool to calculate work function for asymmetric slabs.
If dipole correction is used, the plot of averaged electrostatic potential will have 2 plateaus and there will be two work functions for 2 surfaces of the slab respectively. Two work functions are calculated in this case, and the key of returned dict of abacus_cal_work_function is changed. The key work_function is changed to work_function_results, and the value of it is a list of dict, where each dict contains 3 keys:

• 'work_function': calculated work function
• 'plateau_start_fractional': Fractional coordinate of start of the identified plateau in the given vacuum direction
• 'plateau_end_fractional': Fractional coordinate of end of the identified plateau in the given vacuum direction

[ahxbcn]

Update: add a paremeter dipole_correction to abacus_cal_work_function, which determines whether to use dipole correction along the given vacuum direction.

[ahxbcn]

Update: ABACUS can find the vacuum region in the structure automatically using positions along efield_dir. The implementation is in https://github.com/deepmodeling/abacus-develop/blob/be120f114df564662f3423e34fccb9e7f6c5d828/source/module_elecstate/potentials/efield.cpp#L323. So there's no need to find the vacuum region in this tool.