Remove obsolete import statements

[rouille]

Use ModelImmutables object instead to access the geographical mappings in:

• prereise/gather/demanddata/nrel_efs/tests/test_aggregate_demand.py:from powersimdata.network.usa_tamu.constants.zones import abv2state
• prereise/gather/demanddata/nrel_efs/tests/test_get_efs_data.py:from powersimdata.network.usa_tamu.constants.zones import abv2state
• prereise/gather/demanddata/nrel_efs/tests/test_map_states.py:from powersimdata.network.usa_tamu.constants.zones import abv2state, id2abv
• prereise/gather/demanddata/nrel_efs/map_states.py:from powersimdata.network.usa_tamu.constants.zones import (
• prereise/gather/demanddata/eia/demo/eastern_demand_v6_demo/miso/.ipynb_checkpoints/split_miso_into_subareas-checkpoint.ipynb: "from powersimdata.network.usa_tamu.constants.zones import abv2state"
• prereise/gather/demanddata/eia/demo/eastern_demand_v6_demo/miso/split_miso_into_subareas.ipynb: "from powersimdata.network.usa_tamu.constants.zones import abv2state"
• prereise/gather/winddata/rap/rap.py:from powersimdata.network.usa_tamu.constants.zones import id2abv
• prereise/gather/hydrodata/eia/demo/eastern_hydro_v3_demo/.ipynb_checkpoints/eastern_hydro_v3_demo-checkpoint.ipynb: "from powersimdata.network.usa_tamu.constants.zones import (interconnect2loadzone, \n",
• prereise/gather/hydrodata/eia/demo/eastern_hydro_v3_demo/eastern_hydro_v3_demo.ipynb: "from * powersimdata.network.usa_tamu.constants.zones import (interconnect2loadzone, \n",
• prereise/gather/hydrodata/eia/demo/.ipynb_checkpoints/western_hydro_v2_demo-checkpoint.ipynb: "from * powersimdata.network.usa_tamu.constants.zones import loadzone2state, state2loadzone\n",
• prereise/gather/hydrodata/eia/decompose_profile.py:from powersimdata.network.usa_tamu.constants.zones import abv2interconnect, abv2loadzone
• prereise/gather/hydrodata/eia/net_demand.py:from powersimdata.network.usa_tamu.constants.zones import abv2loadzone