Final model update

[samgdotson]

This PR adds several features to the model

1. PTC and ITC policies
2. Technology-specific WACC values
3. Scripts to run an advanced nuclear sensitivity -- if desired.
4. Variable wind turbine capacity factors (i.e., users can now specify the capacity factor for a particular vintage of wind turbine).
5. Updates the sensitivity script to run a new range of growth scenarios.
6. Updates the "no export" starting demand from 136 TWh to 140 TWh to reflect the average value from the last 25 years (rather than a more recent data point from 2019).

[pep8speaks]

Hello @samgdotson! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file scripts/add_electricity.py:

Line 27:1: W293 blank line contains whitespace
Line 30:1: E722 do not use bare 'except'
Line 32:1: W293 blank line contains whitespace
Line 35:1: E722 do not use bare 'except'
Line 78:1: W293 blank line contains whitespace
Line 79:5: E303 too many blank lines (2)
Line 329:1: W293 blank line contains whitespace
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Line 331:17: E303 too many blank lines (2)
Line 332:80: E501 line too long (80 > 79 characters)
Line 342:80: E501 line too long (83 > 79 characters)
Line 548:60: E225 missing whitespace around operator
Line 548:80: E501 line too long (98 > 79 characters)
Line 588:1: W293 blank line contains whitespace
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Line 591:80: E501 line too long (92 > 79 characters)
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Line 597:80: E501 line too long (96 > 79 characters)
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Line 598:80: E501 line too long (99 > 79 characters)
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Line 599:80: E501 line too long (103 > 79 characters)

• In the file scripts/nuclear_sensitivity.py:

Line 12:80: E501 line too long (82 > 79 characters)
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Line 29:1: W293 blank line contains whitespace
Line 32:1: W293 blank line contains whitespace
Line 33:69: W292 no newline at end of file

• In the file scripts/retrieve_costs.py:

Line 52:52: W291 trailing whitespace
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Line 91:44: W291 trailing whitespace
Line 96:1: W293 blank line contains whitespace
Line 97:59: E231 missing whitespace after ','
Line 98:47: E231 missing whitespace after ':'
Line 99:69: E225 missing whitespace around operator
Line 99:78: W291 trailing whitespace
Line 100:80: E501 line too long (83 > 79 characters)
Line 101:69: E225 missing whitespace around operator
Line 101:79: W291 trailing whitespace
Line 102:80: E501 line too long (88 > 79 characters)
Line 103:1: W293 blank line contains whitespace
Line 104:57: E231 missing whitespace after ','
Line 104:70: E231 missing whitespace after ','
Line 104:80: E501 line too long (94 > 79 characters)
Line 105:1: W293 blank line contains whitespace
Line 106:1: W293 blank line contains whitespace
Line 107:5: E303 too many blank lines (2)
Line 107:52: W291 trailing whitespace
Line 108:80: E501 line too long (85 > 79 characters)
Line 109:80: E501 line too long (90 > 79 characters)
Line 158:30: W291 trailing whitespace
Line 159:30: W291 trailing whitespace
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Line 161:26: W291 trailing whitespace
Line 162:37: E231 missing whitespace after ','
Line 163:37: E231 missing whitespace after ','
Line 163:57: E231 missing whitespace after ','
Line 172:80: E501 line too long (89 > 79 characters)
Line 173:80: E501 line too long (89 > 79 characters)
Line 177:31: W291 trailing whitespace
Line 178:17: E128 continuation line under-indented for visual indent
Line 178:39: W291 trailing whitespace
Line 179:17: E128 continuation line under-indented for visual indent
Line 179:30: W291 trailing whitespace
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Line 181:31: W291 trailing whitespace
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Line 183:30: W291 trailing whitespace
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• In the file sensitivity-script.py:

Line 16:28: E262 inline comment should start with '# '

[lshaver]

Looks good, thanks for wrapping all of this up in a final PR!