Feature: support control on precison of charge cube output

[kirk0830]

Reminder

• Have you linked an issue with this pull request?
• Have you added adequate unit tests and/or case tests for your pull request?
• Have you noticed possible changes of behavior below or in the linked issue?
• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

Linked Issue

Fix #3456

Unit Tests and/or Case Tests for my changes

• A unit test is added for each new feature or bug fix.

What's changed?

Now user can use
out_chg 1 [prec]
to control the precision of output. This PR can brings about a enhanced restart behavior, test the example in examples/scf/lcao_Cu:
start from atomic charge (atomic guess with out_chg 1 3)

start from charge cube file with precision 3 (before this PR, with init_chg file and out_chg 1 10):

start from charge cube file with precision 10 (with init_chg file):

Any changes of core modules? (ignore if not applicable)

• Example: I have added a new virtual function in the esolver base class in order to ...