feat(md): add CSVR thermostat for NVT molecular dynamics

[mintleaf84]

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

Implements #6941

Unit Tests and/or Case Tests for my changes

• Added CSVR test case in source/source_md/test/verlet_test.cpp
• Test passed locally: [PASSED] 1 test

What's changed?

Implement the Canonical Sampling through Velocity Rescaling (CSVR) thermostat as described in:
G. Bussi, D. Donadio, M. Parrinello, J. Chem. Phys. 126, 014101 (2007)

Features:

• New thermostat option: md_thermostat = csvr
• New parameter: md_csvr_tau (characteristic time scale, default: 100.0 fs)
• Properly samples the canonical (NVT) ensemble
• Simple implementation with only one parameter

Usage:

md_type nvt
md_thermostat csvr
md_csvr_tau 100.0  # characteristic time scale in fs

Changed Files

File	Change
source/source_io/module_parameter/md_parameter.h	Add md_csvr_tau parameter
source/source_io/module_parameter/read_input_item_md.cpp	Add parameter parsing for md_csvr_tau
source/source_md/verlet.h	Add apply_csvr() function declaration
source/source_md/verlet.cpp	Implement CSVR thermostat algorithm
source/source_md/test/verlet_test.cpp	Add CSVR unit test
docs/advanced/input_files/input-main.md	Add CSVR documentation

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

No changes to core modules. The modification is only in the MD module.