Fix ITU-R P.618-14 section 2.5: raise p_c_g cap from 1% to 5%

[inigodelportillo]

Summary

• ITU-R P.618-14 section 2.5 updated the minimum percentage used for cloud and gaseous attenuation (p_c_g) from 1% to 5%, accounting for the fact that a larger portion of cloud/gas attenuation is already captured in rain attenuation predictions at low time percentages
• atmospheric_attenuation_slant_path in itur/__init__.py was still using the P.618-13 cap of 1% even when P.618-14 was active
• The fix makes p_c_g version-aware: max(5, p) for P.618-14+, max(1, p) for P.618-13 and earlier
• Updated ITURP618-14_A_total.csv test data to reflect the corrected values

Closes #109

Test plan

• All 41 existing validation tests pass (pytest test/ITU_validation_report_test.py)
• P.618-13 total attenuation test still passes (1% cap preserved for v13)
• P.618-14 total attenuation test passes with updated reference data (5% cap)

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