Refactor&Docs: Simplify deepks_bandgap realization and add input value checks.

[ErjieWu]

Linked Issue

Fix some remain problems in #6226

What's changed?

• Simplify and combine the code for deepks_bandgap labels.
• Merge deepks_bandgap=3 into deepks_bandgap=1. Set the function for deepks_bandgap=4 to deepks_bandgap=3.
• Fix a bug for deepks_bandgap=2: When deepks_bandgap_range does not cover HOMO state, the code fails to deal with the last bandgap in range.
• Add input value check for deepks_band_range.
• Fix the mismatching problem for the input parameter name between docs and code. deepks_bandgap_range should be deepks_band_range. Clarify some expressions in docs.