Refactor: charge mixing

[WHUweiqingzhou]

Fix #3363.
Now ABACUS has implemented various charge mixing methods. To avoid too many uses of if...elseif, a more general design is urgently needed. Besides that, possible new method, f.e. #3335 is also needed to be taken into consideration. To this end, I introduce two new local variables _mixing_rho_type_num and _mixing_rho_unit_num.

Notes:

_mixing_rho_type_num represents how many types of density stored in mixing data. f.e., if only mix charge density (including total density and magnetic density), this value is 1; if we also mix kinetic density, _mixing_rho_type_num will be 2.
_mixing_rho_unit_num represents how many units of density in each density type. It is 1 for nspin=1 ($\rho$); 2 for for nspin=2 ($\rho, m$); 4 for nspin=4 ($\rho,m_x,m_y,m_z$); 2 for nspin=4 using angle mixing method ($\rho,|m|$).

List of Changes

1. add local variables _mixing_rho_type_num and _mixing_rho_unit_num in charge_mixing.h
2. replace GlobalV::variables by using local variables in mix_rho_real()
3. refactor Kerker_screen_real() and inner_product_real() by using _mixing_rho_type_num and _mixing_rho_unit_num