Fix: Add a threshold to avoid E_gap(k)=0 for insulators

[LKFEIYI]

This PR is trying to fix an issue in cal_bandgap and cal_bandgap_updw where the bandgap of insulators (e.g., NaCl) was calculated as 0.0 eV. The threshold of 1e-5 is the value implemented in the old method in ABACUS-LTS.

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[mohanchen]

Thanks for your contribution. The PR LGTM.