Feature: support the default as the value of dft_functional when initialize vdw

[kirk0830]

Background

ABACUS supports the automatic inference of dft_functional from pseudopotential, but its value will not be changed even after the inference is sucessful, this does not matter for other modules, but brings trouble to vdw d3 module because its parameter is functional-specific. The vdw d3 module should search for correct set of parameters according to the name of xc.
However, there are users may think it is harmful to the user-friendliness.

What's changed

This change needs some discussion.
In this PR, ABACUS will pop a warning message like

rather than warning_quit with error message, but I am not sure this is really a good idea (at least now users will not get a crash when leaving dft_functional unset explicitly). I need suggestion from other developers.

@mohanchen @QuantumMisaka @dyzheng @1041176461

Linked Issue

Fix #5944

[QuantumMisaka]

@kirk0830 Nice work !
I do consider that user SHOULD EXPLICITLY KNOW which DFT functional was used in specific calculation even follow the default setting. As for vdw_methods, I suggest add one more notice to tell user that vdw_method like d3/d3_bj is functional-dependent, for some user may not familiar with vdw methods.

[dyzheng]

LGTM，I don't have better idea than this.