Fix NPV/solve_IRR/solve_price methods consistent with cashflow table #180

[yoelcortes]

It should work as it should, but we still need to add tests.

from biorefineries import cornstover as cs
cs.ethanol.price = 0.7198608114634679
table = cs.tea.get_cashflow_table()
print(table['Cumulative NPV [MM$]'].iloc[-1]*1e6)
print(cs.tea.NPV)
31341873.413599316
31341873.413599316

from biorefineries import cornstover as cs
cs.ethanol.price = 0.7198608114634679
table = cs.tea.get_cashflow_table()
index = ['Taxed earnings [MM$]', 'Forwarded losses [MM$]', 'Tax [MM$]', 'Net earnings [MM$]']
table[index]
      Taxed earnings [MM$]  Forwarded losses [MM$]  Tax [MM$]  Net earnings [MM$]
2004                     0                       0          0                   0
2005                     0                       0          0                   0
2006                     0                       0          0                   0
2007                     0                       0          0               -29.4
2008                     0                   -29.4          0               -51.3
2009                     0                   -80.8          0                 -28
2010                     0                    -109          0               -11.3
2011                     0                    -120          0               0.618
2012                     0                    -119          0               0.651
2013                     0                    -119          0               0.618
2014                     0                    -118          0                15.5
2015                     0                    -103          0                30.4
2016                     0                   -72.1          0                30.4
2017                  9.96                   -41.7       3.49                48.2
2018                  51.7                       0       18.1                33.6
2019                  51.7                       0       18.1                33.6
2020                  51.7                       0       18.1                33.6
2021                  51.7                       0       18.1                33.6
2022                  51.7                       0       18.1                33.6
2023                  51.7                       0       18.1                33.6
2024                  51.7                       0       18.1                33.6
2025                  51.7                       0       18.1                33.6
2026                  51.7                       0       18.1                33.6
2027                  51.7                       0       18.1                33.6
2028                  51.7                       0       18.1                33.6
2029                  51.7                       0       18.1                33.6
2030                  51.7                       0       18.1                33.6
2031                  51.7                       0       18.1                33.6
2032                  51.7                       0       18.1                33.6
2033                  51.7                       0       18.1                33.6
2034                  51.7                       0       18.1                33.6
2035                  51.7                       0       18.1                33.6
2036                  51.7                       0       18.1                33.6

[yalinli2]

Test added!

[yalinli2]

@yoelcortes thanks for patching up things, I added the test and it passed. I'm attaching the updated Excel sheet in case for the test case and people can check the formula, feel free to close this!

TEA comparison_V03.xlsx

[yoelcortes]

@yalinli2, I greatly appreciate the rigorous tests you added!

By the way, I also added some tests of my own for consistency between solve_price, solve_IRR, and NPV methods. It turns out there was a minor bug that always resulted in solve_price never reaching an NPV of exactly 0. The issue was in the sales fraction during startup (this is fixed now!; see commits).

[yalinli2]

GREAT! I'll delete the npv branch now, glad we got them all fixed!