Refactor implicit solvation model

[LKFEIYI]

This is the refactoring part of the previous PR #6832 (comment).

Now, it seems that the efficiency of the surchem module has been improved by approximately 100%, which likely comes from reducing the creation/deletion of large matrices. As discussed in #6832 (comment), I deleted the local functions and used the unified interface XC_Functional::grad_rho. Redundant variables are also removed.

I merely tested two examples, and they are attached here for validation:
One is the optimization of ethanol in water, namely the example in #6794, where the time for cal_vel decreased from 1607.60s to 772.56s. The other one is the scf calculation of a Ag slab with cooh (adapted from http://bbs.keinsci.com/thread-57694-1-1.html), where the time for cal_vel decreased from 749.44s to 375.28s.
test.zip

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[dyzheng]

LGTM

[mohanchen]

Thank you for your contribution!!