Update interface GMM logic tree
Once bias-corrected versions of the Alaska interface GMMs are implemented (see ghsc/nshmp/nshmp-lib#217 (closed)), update logic tree with 1/6 weight on the three published models and their corrected counterparts:
AG_20_GLOBAL_INTERFACE
KBCG_20_GLOBAL_INTERFACE
PSBAH_20_ALASKA_INTERFACE
AG_20_GLOBAL_INTERFACE_AK_BIAS_CORR
KBCG_20_GLOBAL_INTERFACE_AK_BIAS_CORR
PSBAH_20_ALASKA_INTERFACE_AK_BIAS_CORR