Dielectric: add check and exit code for metallic ground-states

[bastonero]

Fixes #103

After the NSCF step, we try to detect whether the system has
a metallic ground-state. In that case, we stop the cycle, as we
cannot estimate the critical electric field, and the finite field
simulations would not converge anyways.

[codecov]

Codecov Report

❌ Patch coverage is 13.33333% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.24%. Comparing base (3284716) to head (9a6e74f).

Files with missing lines	Patch %	Lines
src/aiida_vibroscopy/workflows/dielectric/base.py	13.33%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
- Coverage   86.67%   86.24%   -0.43%     
==========================================
  Files          34       34              
  Lines        2552     2567      +15     
==========================================
+ Hits         2212     2214       +2     
- Misses        340      353      +13     

Flag	Coverage Δ
pytests	86.24% <13.33%> (-0.43%)	⬇️

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